Preheat the oven to 190 C / Gas 5. Grease a deep 23cm cake tin and line with baking parchment.
In a mixing bowl, combine the butter and sugar and beat until pale and fluffy. Add in the eggs, one at a time and mix until creamy then sift in the flour, cocoa powder and baking powder, stir to mix then add the milk. Pour the mixture into the prepared cake tin.
Bake in the preheated oven for 20 to 30 minutes, or until a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool completely. Decorate as desired, slice and enjoy!
If you want, you can make 2 layers of the cake (by using two 20cm cake tins) and melt chocolate and add it between them.