1.Preheat oven to moderate. Grease and line 19cm x 29cm slice pan; line 30 holes of three (1 tablespoon/20ml) mini muffin pans with paper cases.
2.Make cake according to directions on packet. Place 1 tablespoon of the mixture in each case; pour remaining mixture into prepared slice pan. Bake mini muffins, uncovered, in moderate oven about 15 minutes and large cake, uncovered, in moderate oven about 30 minutes. Stand cakes 5 minutes; turn onto wire racks to cool.
3.Place large cake on board, top-side down.
4.Make the butter cream. Beat butter in small bowl with electric mixer until as pale as possible. Gradually beat in half of the sugar, then the milk and finally the remaining sugar.
5.Tint ¾ of the butter cream red. Halve remaining butter cream; tint one half purple and other half orange.
6.Reserve 1 tablespoon of each of the three colours of butter cream. Spread remaining red butter cream all over large cake; spread tops of muffins with remaining two coloured butter creams.
7.Press licorice straps around sides of large cake to form sides of cart. Cut remaining licorice strap lengthways into three strips; plait to form rope. Using toothpick, attach rope to cart.
8.Using a little of the reserved red butter cream, attach biscuits to cart for wheels.
9.Arrange muffins on cart; using reserved purple and orange butter cream, decorate muffin tops with a number 1, the child’s name or a birthday message.