Cream the soft butter, sugar and cocoa in an electric mixer (paddle attachment) until well mixed. Stop when it just starts to look creamy. If the dough is overwork, the cookies will tend to spread during baking.
Beat in the egg until well combined.
Add the flour and mix on low speed until the dough forms.
If the weather is warm, you may want to wrap the dough in cling film and leave to rest in the fridge for 15 to 30 minutes.
Roll and use cookie cutters to create shapes.
Place the shapes on a tray lined with baking paper and chill for about 30 minutes.
Bake at 180 degrees C for 8 to 10 minutes, depending on size.
The cookies should spring back up when pressed gently.