Rainbow Petal Layer Cake with Swiss Meringue Buttercream

Preheat your oven to 190°C (170°C fan/gas 5). Grease three 20cm round cake tins with butter and line the bases with baking parchment. In a large mixing bowl, beat 340g of softened butter and 340g of caster sugar together until the mixture is pale, light, and smooth.

Gradually add the 6 eggs one at a time, beating well after each addition until fully incorporated. Stir in 1 tsp of vanilla extract. Gently fold in 340g of self-raising flour and 1 tsp of baking powder using a large metal spoon until just combined. Stir in 2 tbsp of milk to loosen the batter to a dropping consistency.

Divide the batter evenly between the three prepared tins. Bake for 20 minutes, or until the sponges are golden brown and a skewer inserted into the center comes out clean. Allow the cakes to cool in their tins for 10 minutes before transferring them to a wire rack to cool completely.

To make the Swiss meringue buttercream, place 5 egg whites and 300g caster sugar in a heatproof bowl set over a pan of gently simmering water (do not let the bowl touch the water). Whisk constantly for 3-4 minutes until the sugar has completely dissolved and the mixture feels smooth between your fingers.