Preheat the oven to 180°C (160°C fan forced). Grease 3 x 18cm round sandwich pans and line the bases with baking paper.
Open the cake box, remove enclosed frosting sachet and reserve for another use. Make up the cake mix as directed on the cake mix box using the eggs, vegetable oil and water. Divide the mixture evenly between the 3 cake pans. Bake in the oven for 22-25 minutes, or until a round bladed knife inserted into the centre of each cake comes out clean. Remove from the oven and cool a little before turning out onto a wire rack to cool completely. Remove the baking paper.
Line a shallow 30cm x 20cm rectangular baking tray) with a sheet of baking paper. Spread the chocolate frosting over the paper, leaving a small border around the edges. Bake for 20 minutes. The frosting will bubble up and create a toffee. Remove from the oven and allow to cool on the tray. Break into shards once cooled.
Spread a third of the vanilla frosting onto the top of 1 cake, leaving a slightly thicker border around the edges and spoon half of the cherry jam into the centre spreading to the thicker edges. (The thicker edge is in place to make sure the jam doesn’t run.) Repeat with 1 other cake, adding more frosting and the remaining jam. Place the 2 cakes on top of each other, frosting-side up, and top with the third unfrosted cake. Using the remaining third of the frosting, frost the top and sides, creating a ‘naked’ look by scraping off any excess frosting around the sides. Dust the top of the cake with the cocoa powder.
Decorate the top of the cake with the frosting shards (you will have some shards left over) and the dark chocolate or cherry truffles and serve immediately or keep for up to 1 week in an airtight container (see Tip).