Preheat your oven to 190C/170C Fan and line a 9x9inch square tin with parchment paper, and leave to the side for now.
In a large bowl, melt the butter.
Add in the two sugars, and whisk for about 2 minutes so the sugar starts to dissolve and the mixture is smooth.
Add in the black food colouring and whisk again.
Add in the egg and the vanilla, and whisk again briefly until smooth.
Whisk/mix together the plain flour, cocoa powder, bicarbonate, salt and cornflour together so it's evenly distributed.
Add in the dry ingredients, the M&Ms (leave back a handful), and mix with a spatula until a thick cookie dough is formed.
Press the mixture into the bottom of the tin and press in the handful of into the top for decoration. Bake the traybake in the oven for 18-20 minutes, until the top of the cookie bake looks ‘dry’ and a skewer comes out basically clean, bar a few crumbs.
Leave the cookie bake to cool for about 10 minutes, then leave to cool fully on a wire rack. Cut the bake into 4×4 for decent sizes, or 5×5 for smaller ones! Enjoy!
Notes
This delicious recipe is based on a recipe by Crunchy Creamy Sweet which I have adapted to grams, and swapped up slightly!
I used oodles of chocolate M&Ms in this recipe, which I bought from M&M World in London - which is how they are so easily colour specific!
You can buy themed M&Ms online, or pick colours out of packets, or try other Halloween chocolates like orange smarties (but M&Ms hold their colour better)
These delicious beings will last in an airtight container for up to 5 days, but honestly, they’ll be eaten before then.
If you want to bake these into cookies you can, but add in 50g more plain flour. They may still spread a smidge.