• Pre-heat your oven to 160°C/140°C fan.

  Line two 20cm round sandwich tins with greaseproof paper or re-usable baking tin liners.
• Melt your milk chocolate (100g) - You can either melt it in the microwave (heat for 30 seconds, stir and then heat for a further 30 seconds. Repeat until the chocolate has melted), over a bain marie, or in a saucepan over a very low heat. Set the chocolate aside to cool.
• Put all of the other cake ingredients into a large bowl (200g self-raising flour, 40g cocoa powder, 230g caster sugar, 230g margarine or butter, 4 large eggs, 1½ tsp orange extract and 2 tsp milk). Beat together (either using an electric mixer on a low speed or by hand) until the ingredients are combined.

  Add the melted chocolate and mix in (again at a low speed) until combined.
• Split the cake mixture between the two tins. Smooth the mixture using a palette knife or the back of a spoon. Bake for 30-35 minutes until a skewer inserted into the centre of the cake comes out clean.
• Once the cakes are cooked, remove them from the oven and leave them to cool in their tins for about 10 minutes before removing them from the tins and moving them to a cooling rack to cool completely.

While the cake is cooling, make the chocolate orange buttercream

• Melt your milk chocolate (150g) and set aside to cool - It is essential that the chocolate has cooled before it is combined with the other ingredients so it doesn't melt the butter.
• Beat the butter (250g) until soft - This can either be done with an electric mixer at a low speed or by hand. Add the icing sugar (250g), cocoa powder (40g) and orange extract (½ tsp) and beat together on a low speed until combined. 
• Slowly add the melted milk chocolate and beat until combined. Your buttercream needs to be a spreadable consistency. If it is a little too thick, beat in a little milk (no more than 1 tsp at a time) until it reaches your desired consistency.

Build the cake

• Place the bottom layer of your chocolate orange sponge onto your serving dish. Spread about ⅓ of the chocolate orange buttercream evenly across the sponge (I like to use a palette knife, but you could use a regular knife or the back of a spoon).
• Add the top layer of sponge. Again spread ⅓ of the chocolate orange buttercream on top.
• Put the remaining buttercream into a piping bag fitted with a large star nozzle (I use a JEM 1B nozzle). Pipe 12 swirls of buttercream evenly spaced around the edge of the cake. I give the piping bag a little wiggle as I squeeze so that the buttercream looks slightly ruffled.
• Place a slice of Chocolate Orange on top of each buttercream swirl and then add some orange sprinkles.

  If you don't fancy piping, simply split the buttercream evenly between the middle and top of the cake and arrange the Chocolate Orange slices around the edge.