Place the oven rack at the bottom of the oven and preheat according to the instructions on the packaging. Line the base of the springform tin with baking paper, place the ring on top and close securely. Lightly grease the sides with butter or baking spray.

Add the cake mix, eggs and whipping cream to a mixing bowl and mix on the lowest speed until well combined. Then beat on the highest speed for 3 minutes until light and airy. Spread half of the batter into the springform tin and smooth the surface. Add the cocoa mix and 1 tablespoon of milk to the remaining batter and beat for another 0.5 minute on high speed until smooth. Spoon the dark batter over the light batter and gently swirl with a fork to create a marbled effect. Just before baking, sieve about 1/4 of the icing sugar sachet evenly over the batter.

Bake the cake at the bottom of the oven for 50 to 55 minutes until cooked through and golden brown. Insert a skewer into the centre; if it comes out clean and dry, the cake is done. Allow the cake to cool on a wire rack without the springform ring and dust with the remaining icing sugar just before serving. If storing for more than one day, cover with a clean tea towel to keep the crust lightly crisp.