Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line a 12" x 9" traybake tin
Make the sponge by mixing the butter and caster sugar in a large bowl until fluffy, ideally using an electric mixer
Add the eggs, orange extract and milk, and whisk until fully incorporated
Gently whisk in the self raising flour, cocoa powder and baking powder, and whisk in until you can't see any flour anymore
Pour the mixture into the tin, smooth it out of needed to spread it out evenly
Bake for 25-30 minutes or until a skewer inserted in the centre comes out clean. Leave to cool fully either in the tin or on a cooling rack
To make the buttercream, use an electric mixer to mix together the butter, cocoa powder and icing sugar until they start to combine, then add the orange extract and milk and mix until smooth. You can add more milk if the buttercream is too stiff
Spread or pipe the buttercream all over the cake
Decorate with the orange zest and chocolate orange pieces
To serve, slice into squares. Store any leftovers in an airtight container in a cool place and eat within 3 days