1.Grease two deep 23cm square cake pans, line bases with baking paper. Prepare cake mix according to directions; divide between prepared pans. Bake in 180°C oven for about 1 hour. Stand cakes in pans 5 minutes; turn onto wire racks to cool.
2.Using a serrated knife, level tops of cakes so that one is 5cm high and the other cake is 6cm high. Cut each cake into four squares.
3.To make butter cream, beat butter in a small bowl with an electric mixer. Gradually beat in half the icing sugar and beat until as white as possible. Add milk and continue beating, then add remaining icing sugar. Beat until a smooth, spreadable consistency.
4.Tint one-third of the butter cream with pink colouring, another third with blue and remainder with yellow.
5.Sandwich a 5cm- and 6cm-high square with some butter cream. Repeat until you have three 11cm cubes. (There will be one piece of each thickness of cake left over.) Spread each block with a different colour butter cream. Arrange blocks on a 35cm square cake board.
6.Cut each musk stick lengthways into four even-size strips; outline all edges of blocks with musk sticks.
7.Pipe melted chocolate numbers and letters on baking paper.
8.Before chocolate sets, sprinkle with sugar crystals; leave to set. Carefully peel numbers and letter off baking paper and position on blocks.