In a large mixing bowl, beat together the Trex and sugar until light and fluffy. You can do this by hand with a wooden spoon, or use a hand-held electric mixer. Gradually add the beaten eggs, beating well between each addition. Stir in the vanilla extract and milk.
Spoon the mixture into the paper cases. Transfer to the middle shelf of the oven and bake for 20-22 minutes, until risen and golden. Cool in the tin for a few minutes, then transfer to a wire rack to cool completely.
To make the icing, beat the butter in a basin until smooth and creamy. Gradually sift in the icing sugar, beating well between each addition. Beat in the vanilla extract and add the food colouring, if using, then use to top the cakes. Add purchased cake decorations, if you like.