4 x 440g packets moist chocolate cake mix
200g sour cola straps
Black licorice strap, 2 lengths twisted red licorice, skull and bone lollies, toy skateboards and bikes, to decorate
550g unsalted butter, chopped, at room temperature
5 cups icing sugar mixture
Black food colouring gel
1⁄2 cup cocoa powder, sifted
1 tblsp milk
Grease two 20cm x 7cm deep square cake pans. Line bases and sides with baking paper, extending paper 5cm above pan edges.
Prepare two cake mixes, one at a time, according to packet directions. Spoon one cake mix into each pan. Smooth over tops.
Cook both cakes in a moderately slow oven (160C) for about 55 minutes, or until cooked when tested. Stand for 20 minutes. Turn out onto wire racks to cool.
Wipe pans clean. Grease and re-line. Repeat with remaining two cake mixes. Reserve icing sachets for another use.
To make frosting, beat butter in a large bowl of an electric mixer until light and fluffy. Gradually add sugar, 1⁄2 cup at a time, beating until smooth. Spoon 1 1⁄2 cups frosting into a bowl. Tint light grey using black gel. Add sifted cocoa with milk to remaining frosting. Beat until smooth.
Using a ruler, measure two of the cakes into thirds. Cut away a third from each cake.
Position remaining two whole cakes, side by side, on a cake board. Using 1⁄4 cup of chocolate frosting for each layer as glue, position both larger pieces of cake on top of whole cakes at either end. Repeat, `gluing’ smaller cake pieces on top at either end. Refrigerate until frosting is firm. Using a large serrated knife, trim cakes into a half pipe as pictured (see step-by-step guide below).
Spread the remaining chocolate frosting over cake, excluding the centre of the pipe. Spread grey frosting over centre.
Cut sour straps into 1 1⁄2cm-wide pieces for bricks. Cut black licorice into thin strips. Insert toothpicks into red licorice for rails. Position bricks, black licorice, rails, and skull and bones lollies on cake, as pictured. Decorate with skateboards and bikes.