Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line two 8" cake tins (that are at least 2" deep)
Make the sponge by mixing the butter and caster sugar together in a mixing bowl until fluffy (around 3 minutes), ideally using an electric mixer
Add the eggs and milk, and whisk until fully incorporated
Add the self raising flour, cocoa powder, bicarbonate of soda and baking powder and whisk in gently just until you can't see any flour anymore
Divide the mixture between the tins, use scales for accuracy if you like
Bake them for 35-40 minutes or until a thin skewer inserted in the middle comes out clean. Leave them to fully cool either in the tins or remove from the tin and place on cooling racks
To make the chocolate buttercream mix the butter on it's own until it's smooth and creamy. Use an electric whisk for best results
Add the cocoa powder, icing sugar and milk. Mix until smooth. If the buttercream is too stiff then you can add a little more milk
If the cakes have domed on top, level them off with a cake leveller or a serrated knife
Put one of the sponges on your plate or cake stand and pipe or spread some of the buttercream onto it
Add the other sponge on top and pipe or spread the remaining buttercream on top
Decorate with your choice of chocolates, sweets, sprinkles or fresh fruit
Leftovers will keep in an airtight container in a cool place for 3 days