Preheat your oven to 180°C (gas mark 4, 350°F) and line two 20cm (8″) sandwich tins with parchment paper. Leave aside for later use.
Place butter and sugar into a large mixing bowl and cream together until light and fluffy.
Add the eggs and vanilla (or orange extract) to the mixing bowl and mix together until evenly distributed.
Add flour and cocoa powder to the mixing bowl and mix together until combined, then add two tablespoons of milk (this will make the mixture easier to stir).
Split the cake mixture evenly between two sandwich tins and bake in the oven for 25 to 30 minutes.Once the cake is baked, leave the cake aside on a cooling rack for it to cool down.
The Chocolate Orange Buttercream
Whilst the cake is baking, it’s time to make the buttercream.Break the chocolate orange apart and place into a large mixing bowl. Melt in the microwave on a medium heat, mix the chocolate every 10 to 20 seconds until completely melted.
Place butter into a clean mixing bowl and whisk until smooth. Then add icing sugar, cocoa powder and vanilla and whisk together until combined.
Add melted chocolate to the mixing bowl and whisk until combined. If the mixture is too hard and stiff add one to two tbsp of milk.
Building The Cake
Once the cakes have cooled down, trim off the tops if they are domed shaped instead of being flat. Then place the bottom sponge onto a plate or serving dish.
Using an offset spatula spread half of the buttercream on top of the bottom sponge, then place the second sponge on top.
Spread the remaining buttercream and smooth over with an offset spatula. Decorate with extra chocolate orange pieces.