Preheat your oven to 190°C or 170°C fan.
Add the cocoa powder to a large bowl. Slowly mix in the boiling water to form a paste.
Beat in the butter and the sugar, followed by the eggs one at a time. Stir in the flour, cocoa powder and baking powder until all the ingredients are well combined.
Spoon into a muffin tin lined with 12 cases. Place in your preheated oven for 15-20 minutes, until just bouncy to the touch. I usually check them after 15 minutes and leave them for another two, but it will depend on your oven.
Take out of the oven and leave to cool in the tin.
Meanwhile, to make the icing, beat together the butter and the icing sugar until combined. Add the cocoa powder and mix it in. Add your milk or cream a tablespoon at a time until it reaches your desired consistency. For a more glaze-like, pourable icing, add a little more milk and cream.
Spread your icing over your cakes and enjoy!