Preheat oven to 180°C (350°F) and line a 24 piece mini muffin tray.
Mix the cocoa and boiling water together until the cocoa has dissolved. Place in the fridge to cool.
Cream the butter and sugar together.
Add eggs one at a time, beating well after each addition.
Mix the flour, baking soda and baking powder together.
With the mixer on, add a ⅓ of the flour mixture to the butter mixture. Then add a ⅓ of the cocoa mixture. Continue alternating between the two, always beginning and ending with flour mixture. Stir in the vanilla extract.
Spoon the mixture evenly between the muffin cases and place in the oven for 10-15 minutes or until a skewer comes out clean and the cupcakes are soft to the touch.
Place them on a tray to cool completely.
Cookies and Cream Buttercream;
Beat the softened butter until creamy.
Slowly add the icing sugar as the beaters are going. If the mixture is too stiff add a bit more milk until it is the right consistency.