Directions

• For the orange sponge:
• Preheat the oven to 180C / 160C fan / Gas Mark 4.
• In a small saucepan, warm the milk over a low heat to bring to room temperature.
• In a mixing bowl, cream together the butter and sugar until light, fluffy and pale.
• Add the beaten egg to the mixture a little at a time, beating the mixture until smooth between each addition.
• Add the orange zest to the mix and beat until combined.
• Sift the flour, salt and baking powder into the bowl and gently beat until combined.
• Pour the milk into the batter and gently beat in until all of the milk has been incorporated.
• Pour the batter into one of your lined tins and bake for 35-40 mins, or until a skewer comes out clean when inserted into the cake. The top of the cake should also spring back when lightly prodded with a finger.
• Leave the cake to cool in their tins for 10 mins, before removing and placing onto a cooling rack to cool completely.
• For the chocolate orange sponge:
• Preheat the oven to 180C / 160C fan / Gas Mark 4.
• Place the butter, chocolate and coffee into a small saucepan over a low heat, stirring occasionally until the butter and chocolate melts. Leave to one side to cool.
• In a large mixing bowl, whisk together the eggs and sugar until combined.
• Add the buttermilk to the mixing bowl and whisk together to combine.
• Add the cooled chocolate mixture to the mixing bowl and whisk together to combine.
• Sift the dry ingredients into the mixing bowl and fold them until they are fully incorporated.
• Pour the batter into one of your lined tins and bake for 1 hour, or until a skewer comes out clean when inserted into the cake. The top of the cake should also spring back when lightly prodded with a finger.
  (Top tip: this sponge and the black chocolate sponge take the same amount of time bake, so it would be worth making the batter for both cakes and baking them at the same time!)
• For the black chocolate sponge:
• Preheat the oven to 180C / 160C fan / Gas Mark 4.
• Place the butter, chocolate and coffee into a small saucepan over a low heat, stirring occasionally until the butter and chocolate melts. Leave to one side to cool.
• Add the buttermilk to the mixing bowl and whisk together to combine.
• Add the cooled chocolate mixture to the mixing bowl and whisk together to combine.
• Sift the dry ingredients into the mixing bowl and fold them until they are fully incorporated.
• Pour the batter into one of your lined tins and bake for 1 hour, or until a skewer comes out clean when inserted into the cake. The top of the cake should also spring back when lightly prodded with a finger.
• For the buttercream:
• In a large mixing bowl, beat the butter until it is light, fluffy and as white as you can make it.
• Sift 1/2 of the icing sugar into the bowl and beat until the icing sugar has been fully combined. Then repeat this for the remaining 1/2 of icing sugar.
• Add the Cointreau to the buttercream and gently beat until combined.
• To ice the cake:
• Reserve approximately 550g of the buttercream in a bowl. This buttercream you will keep white for coating the outside of your cake.
• Divide the remaining buttercream into 6 bowls and use your food colouring to colour them red, orange, yellow, green, blue and purple. There's no need to measure it exactly, but you'll want to have the least amount of buttercream in the red bowl, then use increasing amounts of buttercream in each bowl as you go through the rainbow, so you end up with roughly twice as much purple buttercream to red. This is because you will ultimately be using a lot more of the purple and blue than you will red and orange.
• Place each colour of buttercream into its own piping bag and snip a largeish hole in the end of each, ready for piping on the cake filling.
• Trim the tops off of each cake so that they are all level and of equal height. Pipe a blob of buttercream on the centre of your cake board / stand, then place your black chocolate sponge on this buttercream to fix the sponge in place.
• Pipe a ring of purple buttercream on top of the sponge, forming a border along its edge. Working into the centre of the sponge, pipe a ring of blue buttercream, then green, yellow and orange, finishing off with a blob of red buttercream in the centre.
• Lay the chocolate orange sponge on top of the first one and repeat the icing process as before, laying the orange sponge on top and placing in the fridge for around 15 minutes to fix the buttercream into place.
• Fit a piping bag with a large round nozzle and add the white buttercream to it. Remove the cake from the fridge and, working from the bottom of the cake to the top, pipe rings of the white buttercream to cover the outside of the cake. Use a cake scraper to smooth the icing around the outside of the cake, filling in any gaps that form with a little more buttercream as required.
• Once you have the sides of the cake as smooth as you can make them, pipe a thin layer of white buttercream onto the top of the cake and use a small cranked palette knife to smooth it out. To neaten up the top edge of your coated cake, gently drag any overhanging buttercream in towards the center of your cake using your palette knife.
  Place the cake in the fridge to chill until the buttercream coating is firm.
• For the chocolate shards:
• Warm the white chocolate in the microwave until completely melted, stirring at 30 second intervals to distribute the heat.
  Alternatively, place the chocolate in a heatproof bowl and place the bowl over a pan of lightly simmering water, stirring occasionally until the chocolate has completely melted. Ensure that the bowl doesn't touch the water in the pan during this process.
• Split half of the chocolate into a separate bowl and use the food colouring to colour one bowl light pink and the other light blue.
• Line two baking trays with baking paper and pour the pink chocolate onto one tray and the blue chocolate onto the other, using a palette knife to smooth it out.
• Scatter some sprinkles over both trays of chocolate and place them into the fridge to set slightly.
• After a few minutes, the chocolate should be set enough to be dry to the touch, but still soft enough to slice into without the chocolate snapping. At this point, score the chocolate into triangles and return it to the fridge to set.
• Decorating the cake:
• Transfer your coloured icings into new piping bags, one for each colour, this time snipping a very small end into the end of each.
• Using a pottery ribbon tool (or a sharp knife if you're careful!), carve small circles into the buttercream coating. If the icing is a little soft after this, place the cake back into the fridge until it firms.
• Fill each of the holes with a little of the coloured icing to create a series of different coloured polka dots!
• Fit a piping bag with a large closed star tip nozzle and pipe stripes of any remaining coloured buttercream into this bag, rotating the bag between each colour to keep each one separate. Pipe a little of this buttercream onto a plate until you can see all of the colours come through.
• Holding your piping bag at a 45 degree angle to the top of your cake and approximately 0.5cm above it, pipe in steady, circular motion along the perimeter of the cake to create a swirling border.
• Remove the chocolate shards from the fridge and stick them into the top of the cake, as desired. You can also break some shards into smaller pieces to place along the bottom of the cake.