Dissolve 300 g chocolate, sugar and butter in a water bath. First add the egg yolk, then the beaten egg white, the hazelnuts and the rum into this chocolate mixture (if you don`t like rum, you can just leave it out). Mix everything well, then pour the mixture into a buttered tin.
Bake in a preheated oven at 180 ° C for about 40 minutes.
Liquefy the remaining 100 g of chocolate and garnish the cake with it.