Pre-heat your oven to 160C Fan/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 in a large bowl, ideally using an electric mixer for around 3 minutes
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 or fold in gently 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 its own for a few minutes, then add the, cocoa powder, icing sugar and milk. Mix until smooth. Use an electric whisk or a stand mixer with the paddle attachment for best results. 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. Create a slightly taller ring of buttercream, about 1 inch from the edge of the cake, this will be the "nest" for the mini eggs
Cover the taller ring of buttercream with the crushed flake bars, then place the mini eggs in the centre
Leftovers will keep in an airtight container in a cool place for 2-3 days