Rainbow Sponge Cake

Preheat an oven to 180 C / Gas 4. Grease and flour two 20cm round cake tins.

Beat the egg whites with an electric mixer until frothy, about 1 minute. Add the sponge cake mix, water and rapeseed oil; continue beating for 2 minutes on medium speed.

Divide the cake batter into six separate bowls. Use a cocktail stick to scoop a dab of food colouring into one bowl of batter and stir; add more food colouring, if necessary, to reach the desired shade. Repeat with the remaining colours and bowls of batter.

Scoop spoonfuls of batter into the prepared tins, alternating the colours. Use a cocktail stick to gently swirl the colours for a marbled effect.

Bake in the preheated oven until a cocktail stick inserted into the cake comes clean, about 30-35 minutes. Cool in the tins for 10 minutes before removing to cool completely on a wire rack.