1.Preheat oven to 180°C (160°C fan-forced). Grease and line deep 15cm-round cake pan. Make cake according to packet directions, pour into pan unitl three-quarters full; bake 45 minutes. Stand cake in pan 5 minutes; turn onto wire rack to cool.
2.Meanwhile, for the chocolate butter cream, beat butter in small bowl with electric mixer until as white as possible. Gradually beat in half the icing sugar, cocoa powder, milk, then remaining icing sugar.
3.Position cake on prepared board. Using ruler and toothpicks, mark 11cm circle in centre of cake. Using markings as a guide, cut deep hollow into cake with a small serrated knife.
4.Spread chocolate butter cream all over cake; fill hollow with coins and half of the rainbow choc-chips.
5.To make chocolate shell: grease 2.25-litre (9-cup) pudding steamer with oil; place bowl in freezer 10 minutes. Place melted chocolate Melts in steamer; swirl chocolate to coat inside of steamer evenly. Continue swirling until chocolate begins to set and stops flowing around the steamer; try to keep the chocolate a uniform thickness, particularly at the top edge. Stand until chocolate is almost set. Freeze until chocolate sets completely.
6.Carefully place pudding steamer with set chocolate shell over cake; using hot cloth, briefly rub outside of bowl. Chocolate shell will slip from bowl to completely enclose cake.
7.Using melted milk chocolate, secure remaining rainbow chocchips, mini M&M’s and M&M’s to chocolate shell.
8.Allow birthday child to break chocolate shell open with a toy hammer.