Ideally using a stand mixer with the paddle attachment, mix the butter, caster sugar and light brown soft sugar together until fluffy and paler in colour. This will take 3-5 minutes in an electric mixer (I use speed 3)
Add the egg and vanilla extract and mix in
Add the plain flour, cocoa powder, baking powder, bicarbonate of soda and salt (you can mix them together in a separate bowl first if you like) and mix them in until just combined
Then mix in the milk chocolate chips briefly, just to evenly distribute them
Divide the dough into portions, I weighed the dough out to make each cookie an equal size
Roll the dough portions into balls using your hands, place them on either a lined baking tray, or into a tupperware container lined with baking paper. Chill them in the fridge for 30 minutes
Meanwhile, pre-heat your oven to 180C Fan/400F/Gas Mark 6
After the 30 minutes is up and your oven is pre-heated, place the dough balls onto lined baking trays, leaving space (at least 2") between them to spread. If you don't have enough baking trays, keep the dough in the fridge and bake the cookies in batches
Bake the cookies for 12 minutes, remove from the oven and immediately push half of a Creme Egg gently into each one, then leave to cool on the trays for at least 10 minutes before moving to a cooling rack
Store leftovers in an airtight container and eat within 3 days