Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease 3 x 18cm round sandwich tins and line the bases with baking paper.
Make up the cake mix as directed on the cake mix box using the oil, water and eggs. Divide the mixture evenly between the 3 cake tins. 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 fudge icing over the paper, leaving a small border around the edges. Bake for 20 minutes. The icing 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 icing 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 edge. (The thicker edge is in place to make sure the jam doesn’t run.) Repeat with 1 other cake. Place the 2 cakes on top of each other, icing-side up, and top with the third uniced cake. Using the remaining third of the icing, ice the top and sides, creating a ‘naked’ look by scraping off any excess icing around the sides. Dust the top of the cake with the cocoa powder.
Decorate the top of the cake with the icing shards (you will have some shards left over) and the cherry or dark chocolate truffles and serve immediately or keep for up to 1 week in an airtight container (see Tip).