Preheat your oven to 180℃ (160℃ for fan ovens). Line a large baking tray or two with baking paper.
Add the softened butter to a large bowl with the caster sugar and brown sugar. Use an electric whisk or stand mixer to mix into a thick paste. Then stir in the egg.
Stir in the flour and cocoa powder using a wooden spoon. When it's starting to stick together, use your hands to press it together into a ball of dough.
Set 12 chunks of the Dairy Milk aside (to top the cookies) and chop the rest into small pieces. Add the chopped pieces to the cookie dough.
Divide the dough into 12 pieces and roll each one into a ball. Top each one with a chunk of Dairy Milk.
Place them on your lined tray, leaving 1-2 inches in between for them to spread. 6 cookies per tray should work fine.
Bake the cookies for 11 minutes. They may look a little underdone but this is key to getting a soft and chewy cookie- they'll firm up as they cool.
Let the cookies cool completely before removing them from the tray. Enjoy!