Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease a 26cm x 38cm Swiss roll tin and line the base and sides with baking paper.
Mix the cake mix, oil, water and eggs gently together in large bowl. Whisk (by hand or electric mixer) for 2-3 minutes until smooth and creamy.
Pour the cake mixture into the prepared tin and gently level the surface. Bake for 20-25 minutes or until just firm to the touch and a rounded knife inserted into the centre of the cake comes out clean. Cool in the tin for 10 minutes then turn out onto a wire rack and leave to cool completely.
Meanwhile, make the truffle mixture. Put the chopped chocolate in a heatproof bowl. Pour 150ml of the cream into a saucepan and heat until almost boiling. Pour the hot cream over the chocolate, leave for 5 minutes then stir until you have a smooth glossy mixture. Leave to cool for 30 minutes, then chill in the refrigerator for 10 minutes, until thick but still spreadable.
Place the chocolate cake on a large chopping board, peel off the baking paper and slice the cake widthways into four even-sized strips. Pour the rest of the cream into a medium mixing bowl and whip until holding firm peaks.
Spread half the whipped cream over one strip of chocolate cake with a small palette knife, almost to the edge. Top with a second strip of cake. Gently spread half the chocolate truffle mixture over the top of the second cake strip. Top with the third cake strip and spread over the rest of the whipped cream. Gently press the last cake strip on top. Cover loosely with cling film and chill in the refrigerator for 30 minutes.
Use a teaspoon to drop the remaining truffle mixture into 12 small mounds onto a baking tray lined with baking paper. Chill in the refrigerator for 30 minutes or until firm.
Using a palette knife, gently spread a thin layer of the chocolate icing over the sides of the layered cake to seal in the crumbs (don’t worry if some of the cream or truffle mixture mixes into the icing). Return the cake to the refrigerator for about 30 minutes. Spread the rest of the icing over the sides and top of the cake, smoothing it with the palette knife.
To make the truffles, sift the cocoa powder onto a flat plate. Briefly roll each mound of the chilled truffle mixture into a ball between the palms of your clean cold hands, then roll in cocoa powder to coat.
Arrange the truffles in a line along the top of the cake. Chill in the refrigerator until ready to serve.