1.Preheat oven to 180°C/350°F. Grease a deep 13cm (5¼-inch) round cake pan and two deep 20cm (8-inch) round cake pans; line bases and sides with baking paper, extending paper 5cm above sides.
2.Make cakes according to directions on packets. Pour mixture into 13cm cake pan until three-quarters full; bake cake about 35 minutes. Divide remaining mixture equally between 20cm pans; bake cakes about 40 minutes. Stand cakes in pans 5 minutes before turning, top-side up, onto wire racks to cool.
3.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.
4.Level cake tops. Secure one 20cm cake, cut-side up, to 30cm (12-inch) round cake board with a little butter cream.
5.Tint butter cream blue; spread cake top with ½ cup of the butter cream. Top cake with remaining 20cm cake, cut-side down. Spread cut-side of 13cm cake with ¼ cup of the butter cream; centre, butter-cream down, on 20cm cake stack.
6.Position rollettes on top of stacked cakes; secure with wooden skewer to make tower. Spread remaining butter cream all over cakes.
7.Make a cut from edge to centre of black cardboard; roll cardboard into a cone shape. Secure with sticky tape on inside of cone; position cone on top of cake.
8.Position mint sticks together on bottom of cake for door.
9.Draw five 3cm x 5.5cm and seven 2cm x 3cm windows on a sheet of baking paper. Turn paper over so outlines are underneath. Melt white chocolate; tint yellow; spoon into a paper piping bag. Pipe windows onto baking paper; stand until chocolate sets.
10.Melt milk chocolate; spoon into separate piping bag. Pipe spiders onto sheet of baking paper; stand until chocolate sets.
11.Decorate castle with windows, spiders, Toblerone and spider’s web.
You can find artificial spider’s web at novelty shops. If you can’t find it, use fairy floss or cotton wool instead. Be sure to remove spider’s web or cotton wool and skewers before cutting and serving the cake. You will need approximately 3 x 100g bars of Toblerone for this cake.