Rainbow Bake with Briony Williams

For The Cake
600g unsalted butter, room temperature
600g caster sugar
600g self-raising flour
1.5 tsp baking powder
12 medium eggs, beaten
Gel food colouring: red, orange, yellow, green, blue, purple

Icing
170ml egg whites (about 7 eggs)
680g icing sugar
680g unsalted butter, softened
1 tsp vanilla extract

To finish
Rainbow sprinkles
Rainbow sweets

1. Oven on to 160°C fan/180°C. Grease and line 6 20cm cake tins.

2. Make the sponges. Beat the butter until pale and smooth. Add the sugar and beat until combined. Add half the flour and baking powder, mix to combine. Add half the eggs, mix to combine. Repeat with the remaining flour and eggs.

3. Divide the batter into 6 bowls. Colour one with red, one with orange, one with yellow, one with green, one with blue and one with purple. Transfer to the prepared tins. Bake for 20-25 minutes until risen and a skewer comes out clean. Set aside to cool.

4. Make the icing. Mix together the egg whites and icing sugar. Using a stand mixer or electric whisk, beat the mixture for 5 minutes until thick and glossy. On a medium speed, add the butter one tablespoon at a time until it is all incorporated then turn the speed up to high and beat for 5-10 minutes until silky smooth. Add the vanilla extract and mix in. Chill for 30 minutes.

5. Build the cake. Place a small blob of icing on your plate/cake board then put the purple sponge on top, press down on the sponge to secure to the board. Add a big dollop of icing on top, smooth out, then repeat with the rest of the sponges. Cover the cake in a thin layer of icing and smooth out as much as possible. Chill the cake for 30 minutes to an hour.

6. Retrieve the cake then cover in the remaining icing (place 3-4 tablespoons of icing in a piping bag with a star nozzle for later). Use a cake scraper to smoothen the sides of the cake and a stepped palette knife to flatten the top. Chill for a further 30 minutes.

7. Cover the top and base of the cake with rainbow sprinkles. Pipe 8 swirls on top then place a rainbow sweet in each swirl.