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 (or baking spread) and caster sugar together in a mixing bowl until fluffy (around 3 minutes), ideally using an electric mixer
Add the eggs, orange extract and milk, and mix until fully incorporated
Add the self raising flour, cocoa powder and baking powder and whisk or fold 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 30-35 minutes or until a skewer inserted in the middle comes out clean. Leave them to fully cool either in the tins or remove from the tins 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 or stand mixer with the paddle attachment for best results
Add the cocoa powder, icing sugar, orange extract 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 sharp 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 the Terry's Chocolate Orange pieces, and orange sprinkles if using
Leftovers will keep in an airtight container in a cool place for 2-3 days