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Method

1. Preheat the oven to 170C/150C Fan/Gas 3. Grease 6 x 20cm/8in loose bottom sandwich cake tins (approximately 4cm/1½in deep) and line the bottoms with greaseproof paper. Dust the tins with flour, then shake out the excess.

2. Beat the butter and sugar together in a stand mixer or with a handheld electric whisk on a high speed until pale and fluffy. Turn the mixer to low speed and add the eggs bit by bit, mixing on high speed after each addition. Add the vanilla, then add half of the flour. Mix until combined, then add the rest of the flour and mix until you have a smooth cake batter.

3. Divide the batter equally between 6 medium-sized bowls and add dashes of each of the food colourings. Mix until you have a pink, yellow, orange, green, blue and purple batter. Pour each colour into one of the prepared tins and bake for approximately 25 minutes, turning after 18 minutes so they colour evenly. Once baked, cool the cakes on a wire rack, then wrap in cling film and store in the fridge.

4. To make the buttercream, place the salted butter in the bowl of an electric mixer. Sift the icing sugar on top, bring the mixture together at a low speed, then beat at high speed for a few minutes until the mixture is pale and fluffy. Add the vanilla extract and beat to combine.

5. Remove the cakes from the fridge and trim the domes off so they are flat. Discard the off cuts. Place the 18cm/7in non-slip mat on the turntable and put the 20cm/8in cake board on top. Put a dab of buttercream on the cake board and place the purple sponge on top. Spread a ¾cm/½in thick layer of buttercream on the top, then place the blue sponge on top. Repeat this process for the green, yellow, orange and pink sponges until you have assembled the whole cake with five layers of buttercream in between. The final top sponge should have the base facing upward. Gently press the top of the stack to ensure the cake is level and free of air bubbles. Chill in the fridge for 10 minutes.

6. Using a knife or a cheese grater, trim the sides of the cake to within a few millimetres of the cake board it’s sitting on. Add a splash of milk to the buttercream and beat for 30 seconds to loosen. Put 2 spoons of the buttercream on the top of the cake, then spread the buttercream towards the edges and down the sides with a palette knife. Scrape of any excess and discard. This layer should be very thin, almost translucent in parts. Return the cake to the fridge for 10 minutes.

7. Divide the remaining buttercream into 6 equal portions and place in bowls. Colour the buttercream in the same way you coloured the cake batter, then place each colour into a separate piping bag.

8. Put the 25cm/10in non-slip mat on the turntable and place the 30cm/12in cake board on top, along with the cake. Pipe concentric rings of each colour buttercream around the outside of the cake, in the same order as the picture. Add enough pink buttercream to cover the top of the cake. Scrape the excess buttercream off the sides of the cake using the side scraper. Turn the turntable in a fluid motion while ensuring you keep the blade at a 45 degree angle against the side of the cake with your other hand. Repeat until you have a nice smooth finish.

9. Using your palette knife, carefully brush any excess buttercream that’s gathered at the rim of the cake towards the centre, aiming all the time for sharp edges, straight sides and a flat top. If the buttercream has ‘torn’ in places, dip your palette knife in hot water and use that to smooth over any imperfections. Put the cake back in the fridge for 10 minutes, or until you’re ready for the next stage.

10. Put the cake directly on the turntable. Pipe 6 swirls on top of the cake with your remaining colours and sprinkle them with the multi-coloured sprinkles.