Preheat the oven to 180C/350F/Gas 4. Grease and line two 20cm/8in sandwich tins.
Melt the chocolate in a bowl set over a pan of simmering water.
Meanwhile, cream the sugar and butter together in a bowl until light and fluffy. Add the orange zest and juice and the melted chocolate. Slowly beat in the eggs, one at a time, until well combined.
Sift the flour, baking powder and cocoa powder into the mixture and fold until well combined. Divide the cake batter between the two sandwich tins and bake for 25-30 minutes, or until cooked through.
Beat the butter until pale and fluffy. Add the icing sugar a little at time until fully incorporated.
Add in the orange food colouring and orange extract then mix until fully incorporated. Then fill a piping bag with a circle piping nozzle and pipe onto the first layer of cake.
Once the bottom layer is fully covered with icing sugar, leave it for a minute to give the buttercream to harden a little bit. Then add the second layer on top of the buttercream gently, pipe more icing on to the top layer of the cake. Add a couple of slices of terry chocolate orange and enjoy.