Melt the butter in a saucepan and bring it to a boil while constantly stirring. Add 4-5 tablespoons of whipping cream and stir with a whisk. Add sugar and vanilla sugar or vanilla extract to it. Once the sugar is dissolved (3-5 minutes cooking time) add the ground hazelnuts first and then the chopped almonds. Stir with a wooden spoon until the ingredients are well mixed. Take the saucepan off the heat and let the mixture cool down.
Cover a baking sheet (17 x 11 inches) with baking paper. (Add an extra 10 cm 4 inches on the shorter sides so that the paper overlaps the baking sheet. Fold the overlap of the paper, so that it stands up on the sides. It makes it easier to move the cake after baking from the sheet). Form the shortcrust into a roll in the length of the baking sheet and place it onto the covered baking sheet. Roll out the dough using a rolling pin. Pass the apricot preserve through a small sieve and spread it even onto the top of the dough. Spread the caramelized nut mixture on top of the apricot preserve. Gently press the nut mixture onto the dough using the back of an angled spatula.
Preheat the oven to 180°C (360°F) and bake the cake on the middle roast for 22-25 minutes. When the cake has finished baking, lift it with the paper on a cooling rack and let it cool down for 20 minutes. Cut the cake in 15 equal squares. Cut the squares in diagonal into triangles.