Preheat your oven to 180℃ (160℃ for fan ovens) and line a large tray or two with baking paper.
To a large bowl, add the butter, caster sugar and soft brown sugar. Mix with an electric whisk or stand mixer to create a thick paste.
Add the egg and mix again until smooth.
Next, add all of the flour and the cocoa powder. Mix it in with a wooden spoon and once it's starting to stick together, use your hands to press it into a ball of dough.
Gently knead the chopped Double Decker bars and the chocolate chips into the cookie dough.
Divide the dough into 12 pieces and roll each one into a ball. Gently press them down to create discs that are around 1-inch thick.
Pop them onto your tray(s), leaving an inch or two of space in between each one to allow for spreading.
Bake the cookies, one tray at a time, for 11 minutes exactly. They will look a little underbaked at first but will firm up as they cool. Do not keep baking them or they'll end up hard and crispy!
Leave them on the tray until completely cool, then enjoy.