Preheat the oven to 180°C (160°C fan forced). Grease 4 x 20cm round cake pans and line the bases with baking paper. Make up the cake mixes as directed on the boxes using the butter, milk and eggs. Divide the mixture equally among the cake pans and bake for 23-28 minutes or until a round bladed knife inserted into the centre of each cake comes out clean. Cool slightly in the pans, then turn out onto wire racks to cool completely. Remove the baking paper.
When cooled, trim the tops of the cakes to make them level, if necessary. Place one cake on a cake turntable and spread with 125g of the vanilla frosting. Repeat to sandwich all the cakes together with 125g of vanilla frosting between each layer. Ensure each cake is sitting evenly on top of the one underneath.
Spread a further 275g of the vanilla frosting in a thin layer over the top and sides of the cake, smoothing with a palette knife (this is called the crumb coat and gives a smooth surface to apply the final coat of frosting). Chill in the fridge for 30 minutes or until firm to the touch.
Spread the remaining 300g of the vanilla frosting over the top and sides of the cake. Drag a cake icing smoother or long palette knife around the icing on the side of the cake, keeping the turntable rotating to get a smooth finish (you may still be able to see some of the chocolate cake through the icing). Chill for a further 30 minutes.
Empty the two sachets of chocolate frosting from the cake mix boxes into a microwave proof jug or bowl. Microwave on Medium in 10 second bursts, stirring after each burst, until the frosting is smooth and runny.
Using a teaspoon, spoon some of the frosting all around the top edge of the cake, allowing it to drip down the sides. Leave the remaining frosting for 5-10 minutes until slightly thickened, then spoon into the centre of the cake and spread it out to the frosting at the edge. Use the tip of a small palette knife to create a ridged pattern in the chocolate frosting on top of the cake. Leave in a cool place for 30 minutes.
To decorate, roughly crush one of the mini candy canes. Arrange the remaining candy canes and the halved chocolate sandwich biscuits on top of the cake. Scatter the crushed candy cane around the decoration.