For the toffee tart, first prepare the filling. To do this, coarsely chop the nougat and place in a heatproof mixing bowl. Heat the whipped cream in a small saucepan until bubbles form. Pour over the nougat and let stand for 5 minutes.
Stir until smooth and refrigerate. It is best to do this the day before.
Preheat the oven to 180 °C and grease a baking pan. Beat the eggs with the sugar and vanilla sugar until very foamy. Mix the ground hazelnuts, flour, baking powder and cocoa and gently fold into the egg mixture.
Pour the batter into the baking pan, bake for about half an hour and test with a knitting needle if the batter still sticks. Do not remove the dough from the oven until it remains clean.
Once the dough is baked, place on a cooling rack, remove from the pan after a few minutes and let cool. This can also be done the day before.
For the filling, whip the nougat topping and vanilla sugar with a food processor on high speed until light and smooth.
Add the mascarpone, a little at a time, and whip until fluffy. Cut the cake in two and set aside.
Now spread a third of the filling on the first part and place the second part of the cake on top and spread again with the second third of the filling, now add the third part of the cake on top.
Spread a thin layer of the remaining cream all around the cake.