Preheat the oven to 180°C (160°C Fan, Gas Mark 4). Grease a 24cm (9 1/2inch) diameter ring cake tin, with melted butter using a pastry brush, Sprinkle the greased tin with flour ensuring all the inside of the tin is coated.
In a large grease-free bowl whisk the egg whites and Cream of Tartar until very stiff – as if making a meringue mixture. Add half the sugar and the Vanilla Extract and whisk until thick and glossy.
Sift the flour and remaining sugar, in 4 batches, on to the egg whites, folding in gently but thoroughly after each addition.
Spoon a quarter of the mixture into another bowl and gently mix in a few drops of Pink Colour Gel.
Drop large spoonfuls of the plain mixture and smaller spoonfuls of the pink mixture into the tin, and gently swirl together to give a marbled effect. Smooth the top and bake for 25 minutes until risen, firm to the touch and lightly golden. Turn on to a wire rack to cool.
To decorate, sieve the icing sugar into a bowl. Add the Vanilla Extract and mix in 4-5 tsp of warm water to make a smooth, glace icing. Drizzle over the top and sides of the cake and decorate with the sprinkles. Leave to set for a few minutes before slicing to serve.