Halloween cookie bars take one tin and 20 to 25 minutes in the oven: cream the butter and sugars, beat in an egg, fold through a cocoa dough, stir in Halloween M&Ms and bake at 180C. The catch is the colour, because black cookie dough has a habit of baking to a flat grey, or turning bitter when too much colouring goes in.
Cocoa fixes both problems, so half a teaspoon of colour is enough and the centre still gives when you bite. That is how to make Halloween cookie bars that stay dark after baking.
Ingredients for Halloween Cookie Bars
Twelve ingredients make 16 squares, with the main ones weighed in grams for Irish kitchens.
Halloween Cookie Bars
Prep: 15 minutes · Bake: 20 to 25 minutes · Cooling: 1 hour · Makes: 16 squares · Tin: 20 cm square baking tin
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115 g butter, softened to room temperature
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55 g granulated sugar
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135 g light brown sugar
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1 medium egg
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1 teaspoon vanilla extract
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1/2 teaspoon black gel food colouring
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200 g plain flour
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50 g cocoa powder
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1 teaspoon bicarbonate of soda
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1 tablespoon cornflour
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1/2 teaspoon salt
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300 g Halloween M&Ms, with 50 g kept back for the top
Sugar and butter set the texture: creaming them together gives the bars their tender edges. Bellingtons light muscovado sugar stands in for light brown sugar and adds a soft, toffee-like note that suits chocolate, while the granulated sugar stays plain. Connacht Gold butter is an Irish butter that works well for baking. Unsalted gives you full control of the seasoning, so if your block is salted, cut the added salt to a pinch.
Most quiet bake failures start with the dry ingredients. The deep base colour comes from Green & Black's cocoa powder. Doves Farm plain flour gives the bars enough structure to lift out of the tin in clean squares, while bicarbonate of soda, called baking soda in American recipes, stops them baking into a dense brick.
Watch the cornflour. Suma cornflour is the fine white thickening powder, the same thing American recipes call cornstarch, and not cornmeal. A tablespoon keeps the crumb soft on day three. A half teaspoon of West of Dingle salt sharpens the chocolate.
A teaspoon of Nielsen-Massey vanilla extract rounds out the cocoa.
The orange, green and purple of Halloween M&Ms against black dough give the bars their look. No black gel colouring? The cocoa alone still gives a deep, dark bake. If you can't find the M&Ms, any candy-coated chocolate in autumn colours is a fair substitute, and so are Smarties, the candy-coated chocolate discs sold across Ireland.
How to Make Halloween Cookie Bars Step by Step
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Preheat the oven to 180C (160C fan, gas mark 4). Line a 20 cm square baking tin with parchment paper, leaving a little hanging over two sides so you can lift the bars out later.
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Beat the butter and both sugars with a mixer for about three minutes, until pale and fluffy. Scrape down the bowl with a spatula halfway through. No mixer? A wooden spoon works if the butter is soft, it just takes a few minutes longer.
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Beat in the egg, vanilla and black gel colouring until the mixture is an even dark grey.
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In a separate bowl, whisk the flour, cocoa, bicarbonate of soda, cornflour and salt together so there are no cocoa lumps.
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Tip the dry ingredients into the butter mixture and fold with a wooden spoon until you can no longer see flour. Stop there. The mixture should be thick, closer to cookie dough than cake batter.
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Fold in 250 g of the M&Ms.
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Press the dough into the tin in an even layer, using the back of a spoon or your fingertips. Press the remaining 50 g of M&Ms into the top so they show.
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Bake for 20 to 25 minutes.
Black dough hides the usual golden brown cue, so judge doneness by the edges. They should look set and start to pull away from the tin, while the centre still looks a touch soft.
How to Get Properly Black Cookie Dough
Get it black in two layers: cocoa for the base and a small amount of gel to finish. Fifty grams of cocoa turns the dough deep brown on its own. The gel only has to push it the last stretch to black.
Use gel or paste colouring rather than liquid. Liquid colouring is diluted, so reaching black means adding enough to loosen the dough and dull the chew. Gel is concentrated.
Expect the dough to look charcoal, not jet black, when it goes into the tin. Gel colour tends to deepen as it sits and bakes. Resist adding more. Extra colour brings the bitterness.
If you have no black colouring, the cocoa alone bakes to a dark chocolate brown that still looks spooky next to bright M&Ms. Building black from red, blue and green is not worth the effort. It takes so much that the dough turns sticky and the colour goes muddy.
One tip for party hosts: very dark dough can leave lips and fingers slightly stained. Put out napkins.
How Long Do Cookie Bars Need to Cool Before Slicing?
Let the bars cool completely in the tin, which takes about an hour. That wait is the difference between neat squares and a gooey collapse. A centre that looked slightly underbaked in the oven finishes setting as it cools. Cutting early undoes that.
Lift the slab out by the parchment overhang, set it on a board and cut with a sharp knife, wiping the blade between cuts so the chocolate doesn't drag. Sixteen squares suits a lunchbox. Smaller pieces are easier to hand round at Halloween parties.
Why Cookie Bars Turn Out Cakey and How to Fix It
Cakey bars almost always trace back to flour, mixing or time in the oven.
Flour is the usual culprit. Weigh 200 g, because a cup scooped from the bag packs down and weighs more than you intend, and every extra spoonful pushes the bars from chewy towards dry. Overmixing does the same damage: once the flour goes in, fold only until the streaks disappear, since beating further builds a bready crumb. Timing catches people out too. Ovens vary, so start checking at 20 minutes, because bars that look fully baked in the oven are overbaked by the time they cool.
How Do Cookie Bars Differ From Cookies and Blondies?
Cookie bars are cookie dough pressed into a tin and baked as one slab, where cookies are scooped and baked individually. Blondies share the slab format but are cocoa-free, built on brown sugar and closer to cake, while brownies lean on chocolate and bake fudgy. The slab gives you more soft middle and less crisp edge than a cookie.
In Ireland, anything baked in a tin and cut into squares often gets called a traybake. A traybake usually means a frosted sponge though, so search for cookie bars when this is the style you want. Barmbrack is still the traditional Irish Halloween bake, and cookie bars are the newer arrival on the table.
What Can You Add to Halloween Cookie Bars?
The dough takes almost any mix-in, as long as the total stays around 300 g.
For loaded cookie bars, chop leftover Halloween candy bars and fold them in alongside half the M&Ms. Reese's Pieces, the peanut butter candy pieces, add a salty crunch, though tell your guests if peanuts are involved. Crushed Oreos, the chocolate sandwich biscuits, give a cookies and cream look, and chopped dark chocolate or chocolate chips cut through the sweetness.
Sprinkles and the reserved M&Ms make the top more festive: scatter orange and purple sprinkles over it before baking. Candy corn works pressed into the top too, and our guide to American Halloween sweets covers what it is and what else is worth trying.
How to Store Halloween Cookie Bars
Keep the bars in an airtight container at room temperature, with a sheet of parchment paper between the layers so the M&Ms don't stick. They stay at their best for three to four days. The fridge is not needed, and it makes them firmer, not chewier.
For longer, wrap squares individually and freeze for up to a month, then thaw on the counter.
Shop Baking Ingredients for Halloween Cookie Bars
Plain flour, cocoa powder, bicarbonate of soda and cornflour all sit together in our baking collection. Have a tin of properly black, properly chewy bars cooling before the first trick-or-treaters knock, and the Halloween dessert is sorted.
