1.Preheat oven to 180°C (160°C fan-forced); grease and line two 20cm ring pans. Make cake according to directions on packet, divide mixture between pans; bake about 35 minutes. Stand cakes in pans 5 minutes; turn onto wire rack to cool. Using serrated knife, level cake tops so cakes are the same height.
2.Place cakes cut-side down. Cut segments from cakes, as shown.
3.Assemble cake pieces on board, cut-side down, to form number; discard remaining cake.
4.To make butter cream; beat butter until white as possible, gradually beat in half the sifted icing sugar then milk, then remaining icing sugar. Beat until smooth.
5.Tint butter cream with blue colouring; spread all over cake.
6.On surface dusted with icing sugar, knead icing until smooth. Knead green colouring into icing; roll until 3mm thick.
7.Using paper pattern and a sharp-pointed knife, cut five lily leaves from icing. Using same knife, gently mark veins on leaves; drape over rolling pin to dry. Position leaves on cake with water lily and chocolate frogs.
Transpose the leaf pattern onto cardboard, then use the cardboard leaf as a guide when cutting the leaf shapes from icing. The leaves can be made up to 3 days ahead; store in an airtight container. Using the leaf pattern, cut five lily leaves from the icing then drape over a rolling pin to dry. Place the two ring cakes cut-side down. Cut segments out of the ring cakes, as shown. Assemble the cake pieces on board to form the number; discard remaining cake pieces.