Preheat the oven to 180°C/fan 160°C/gas mark 4 and line a muffin tin with cases.
Cream 225g of butter and the golden caster sugar together until light and fluffy. Add the eggs one by one, mixing well in between. Finally fold in the flour, cocoa and baking powder until well combined.
Spoon the mixture evenly into the muffin cases and bake in the oven for 25 minutes or until the cupcakes are springy to the touch and a skewer comes out clean.
To make the buttercream use an electric or hand mixer and whisk the golden icing sugar and the rest of the butter together until you get a breadcrumb texture. Add the milk and mix on high speed until light and fluffy. Add a few drops of green food colouring to the buttercream and mix until you get a ‘Christmas tree’ green colour.
Pipe the buttercream onto the cakes in a pyramid shape and top the ‘trees’ with festive sprinkles for the perfect finishing touch.