Preheat oven to 175C fan-forced and grease and line a loaf tin so that the paper extends over the long sides.
Using an electric mixer, beat the butter and sugar together until creamy then beat in the egg.
Alternating between the flour and milk, add half of each at a time and beat just until combined.
Divide the batter into three bowls. To one, add a few drops of pink colouring and the strawberry essence. To another, add the vanilla extract and to the last, mix in the cocoa.
Using a tablespoon of batter at a time and alternating between the three, spoon the batter into the tin. Use a skewer or knife to marble the batter then bake for 30-35 minutes or until a cake tester inserted in the middle comes out clean.
Leave to rest in the tin for 5 minutes before transferring to a wire rack to cool completely.
Make the icing according to the instructions on the packet and add pink colouring. Ice the cake and dust with icing sugar to serve.