Preheat oven to 180C/350F. Line 18 holes of two 12-hole deep flat-based patty pans with paper cases.
Beat butter, extract, sugar and egg in a small bowl with an electric mixer until light and fluffy. In two batches, stir in desiccated coconut, sifted flour and baking powder, sour cream and milk. Divide the mixture among paper cases.
Bake the cakes for about 20 minutes. Stand cakes in pans for 5 minutes, then transfer, top-side-up, onto a wire rack to cool.
Meanwhile, make coconut ice frosting. Sift icing sugar into a medium bowl; stir in desiccated coconut and egg white. Place half the mixture in a small bowl and tint with food colouring.
Drop alternate rounded teaspoons of white and pink frosting onto cakes; marble over the top of each cake.