Set the oven to 175 degrees.
Melt butter over low heat. Stir in the cocoa powder. In a bowl, mix the sugar, flour, vanilla sugar and salt. Add the butter and then the eggs one by one.
Line the bottom of a 24cm round tin with parchment paper. Pour the batter over it and bake on a baking sheet in the middle of the oven for 20 minutes.
When the cake is cooked, place the mold directly on the rack and make the chocolate icing.
Boil the cream, sugar and syrup in a saucepan. Then remove the pan from the heat and add the coarsely chopped chocolate. Stir with a spatula until all the chocolate is dissolved. Return the pan to the heat and simmer for approx. 20 minutes until you see that the mass has started to thicken a little. Then remove the pan from the heat and stir in the butter. Continue to stir until all the butter is completely blended. Pour the frosting over the base of the pie. Leave the cake on the kitchen counter until it is at room temperature. Put it in the fridge for at least 2 hours.
Before serving, carefully loosen the cake from the edge of the pan with a sharp knife and place it on a thin cake pan. If you want to decorate, you can put some nuts and lightly roasted raspberries in a row on the cake.
The cake will keep in the fridge, but leave it at room temperature for an hour before serving so that it becomes extra good and “messy”.