Pre-heat the oven to 175°C/350°F (convection). Melt chocolate and butter in a pan set over low heat. Stir occasionally to avoid burning. As the cake will be very sweet due to the large amount of sugar, I suggest you use very dark chocolate (with a high cocoa content).
In the meantime, whisk sugar, eggs, vanilla sugar, and salt in a bowl. Only whisk until everything is just combined and small bubbles appear (which you can only see when you stop whisking).
Now add the butter-chocolate mixture and whisk briefly.
Finally, add flour and whisk again until batter is homogenous. Pour batter onto a greased baking sheet and distribute evenly. Now bake for approx. 15 min. at 175°C/350°F (convection).
As soon as you remove the cake from the oven, decorate with sprinkles (otherwise they will not stick at all), and also slice when warm.
If you want to add some fruit as a special treat, I would recommend fresh strawberries.