John Whaite's pride rainbow cake

Preheat the oven to 180˚C, gas mark 4. Grease and line the bases of 6 x 20cm sandwich cake tins (or as many tins as you have, and bake in batches).

For the cake batter, beat the baking spread and sugar together (preferably in a freestanding mixer with a paddle attachment) until pale and fluffy. Add the eggs one at a time, beating well after each addition. Don’t worry if the batter curdles.

Sift the flour and salt into a separate bowl, then add 1/3 to the butter mixture and beat in. Add 1⁄2 the buttermilk, followed by another 1/3 of the flour mixture, then the remaining buttermilk and finally the remaining flour, beating after each addition. When you have a smooth batter, divide it as evenly as possible between 6 bowls (about 435g batter in each).

Add enough of each food colour to the bowls to create vibrantly coloured batters, mixing a little blue and red for the purple layer. Pour each one into a prepared cake tin and bake for 20-25 minutes, until a skewer inserted into the centre comes out clean. Cool in the tins for 10 minutes, then invert onto wire racks to cool completely. If baking in batches, repeat with the remaining batter.

For the Italian meringue buttercream, put the egg white and cream of tartar in the bowl of a freestanding electric mixer fitted with a whisk attachment. Put the sugar in a saucepan with 100ml water. Bring the sugar to the boil and cook, without stirring. When the sugar reads 110˚C on a sugar thermometer, turn the mixer to medium speed and beat the egg whites, stopping once they reach medium peaks. When the sugar syrup is at 118˚C, turn the mixer back on to medium speed and slowly pour in the hot syrup – be sure to pour it down the side of the bowl to prevent it from catching the whisk. Increase the speed to high and beat for about 20 minutes, until thick, glossy and at room temperature.

Once the meringue has cooled, add the butter – which should be soft but not greasy – one cube at a time, while whisking on a high speed. As you add it, the meringue will deflate and start to look very soupy. Don’t panic, this is all part of the process. As you add the last few cubes of butter, the buttercream should thicken and become smooth and aerated. Once it does, use a spatula to scrape down the sides and bottom of the bowl, then add the salt and vanilla seeds and whisk for a couple more minutes until evenly distributed.

To assemble the cake, put a 20cm coated cake card (if available) onto a cake turntable (preferably) or cake stand. It helps to secure the card down with a damp piece of kitchen paper. Smear 1 tsp of the buttercream in the centre of the cake card, then place the purple cake on top. Spoon a small amount of buttercream on top of the cake (just enough to cover the top thinly – if you want to be exact, I use 3 x 42g ice-cream scoops). Spread the buttercream out over the cake with a small crank-handled palette knife and level it off as evenly as possible. Top with the blue layer and repeat with more buttercream, then the green, yellow, orange and red layers.

Spread a very thin layer of buttercream around the outside of the entire cake to act as a ‘crumb coat’ – this will trap any pesky crumbs and stop them from getting everywhere. Chill the cake for 30 minutes, then spread the remaining buttercream over the top and sides, getting it as smooth as possible with an offset palette knife.

Carefully pat some of the sprinkles around the base of the cake – I find it helpful to balance the cake card on a tumbler over a tray, then gently press palmfuls of sprinkles into the buttercream. Chill the cake for 1 hour to firm up before you add the chocolate drips.

Melt the chocolate with the oil in a heatproof bowl over a pan of barely simmering water, stirring occasionally to ensure the chocolate is smooth and runny. Pour the chocolate into a piping bag and pipe drips around the edge of the cake, using different pressure for each drip so the drips are different lengths. Allow the chocolate to set fully.

Once the chocolate has set, dust the drips with gold lustre. The best way is to unscrew the bottle and tip a little bit of the lustre into a shot glass or espresso cup. Use a small brush to carefully dust each drip with lustre until it shines. Finish with a few more sprinkles on the top, outer edge of the cake.