Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line three 8" cake tins (that are at least 2" deep)
Make the sponge by mixing the butter or baking spread, peanut butter and caster sugar in a large bowl ideally using an electric mixer
Add the eggs and mix until fully incorporated
Add the self raising flour and 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 skewer inserted in the middle comes out clean. Leave them to fully cool either in the tins or on cooling racks
If the cakes have domed on top, level them off with a cake leveller or a sharp knife
To make the buttercream, mix the butter on it's own for a few minutes. Then add the peanut butter, cocoa powder, milk and icing sugar, ideally using an electric mixer. You can add more milk if the buttercream is too stiff
Stack the sponges up on your plate or cake stand, piping or spreading buttercream between each layer. If you're using it, drizzle the Reese's Spread or peanut butter over each layer of buttercream, letting it drizzle over the sides. You will need to warm it in the microwave for 20-30 seconds first to get a pourable consistency
When you get to the last layer, pipe larger rosettes all around the edge and smaller ones in the centre. Decorate with the Reese's Peanut Butter Cups and Mini Cups, and sprinkles if using
Serve immediately, store leftovers in an airtight container in a cool place and eat within 3 days