For the Nutella slice, first prepare the dough. For the dough, beat the yolks and sugar until foamy, stir in the oil and water. Fold in the cocoa, the flour with the baking powder and the beaten egg white.
Spread the dough on a baking tray lined with baking paper and bake at 180 °C for about 20 minutes. For the cream, mix the QimiQ for about 1 minute, stir in the cream stiffener and sugar.
Divide the mixture in half, stir the Nutella into one half and spread on the baked cake. Carefully spread the remaining white cream on top and smooth it down.
Sprinkle with chocolate flakes and chill before serving.