1. Grease a 23 cm (9 inch) fluted ring tin or a 20 cm (8 inch) round loose-bottomed cake tin. Preheat the oven to 180°C (350°F or gas mark 4).
2. Put the flour, a pinch of salt and 7 tbsp of caster sugar in a bowl and set aside. Place the egg whites in a large, grease-free bowl and whisk until foamy but not stiff.
3. Add the cream of tartar and 2 tbsp of caster sugar, then whisk until the egg whites form soft peaks. Add the vanilla extract and remaining caster sugar and fold in using a large metal spoon.
4. Sift in the flour and sugar, folding the mixture as you go. Divide the mixture equally between 3 bowls. Gently fold pink food colouring into one mixture. Sift the cocoa into another and gently fold in. Leave the remaining one plain.
5. Transfer the mixtures, a spoonful for each at a time, to the tin. Bake for 25-30 minutes or until the cake is golden and a skewer inserted into the centre comes out clean. Cool in the tin for 20 minutes.
6. Turn the cake onto a plate, dust with icing sugar and serve.