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, peppermint extract 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
Make the green mint buttercream by mixing together the softened butter, peppermint extract, milk and icing sugar until smooth. You can do this by hand, but an electric whisk or stand mixer with paddle attachment will provide the best results. If the buttercream is too stiff, you can add a little more milk
Then add a small amount of green food colouring and mix until a pastel green shade is reached. You can add more food colouring if needed
Make the chocolate buttercream by mixing together the softened butter, cocoa powder, milk and icing sugar, and mix until smooth. You can do this by hand, but an electric whisk or stand mixer with paddle attachment will provide the best results. If the buttercream is too stiff, you can add a little more milk
Fill a piping bag fitted with your chosen nozzle, with the gren mint buttercream on one side and the chocolate buttercream on the other side
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 Aero Bubbles and chunks of Aero Bar
Leftovers will keep in an airtight container in a cool place for 3 days