1.Preheat oven to 180°C. Grease cake pan; line base with baking paper.
2.Make cakes according to directions on packets; pour mixture into pan. Bake about 45 minutes. Stand in pan 5 minutes before turning, top-side up, onto a wire rack to cool.
3.Make butter cream: In a small bowl, beat butter with an electric mixer until as white as possible; beat in sifted icing sugar and milk in two batches.
4.Using a serrated knife, level top of cake. Cut jigsaw-shaped pieces from cake.
5.Keeping colours separate, empty cake sprinkles on to five pieces of baking paper. Work with one jigsaw piece at a time: spread top and the side(s) that will show with butter cream; roll in one of the sprinkle colours then position on cake board; spread a little more butter cream on the un-iced side(s) that will join with the next jigsaw piece. Continue until all pieces are coloured and fitted together.