Melt 250 g chocolate and butter in a double boiler. Separate the eggs and beat the yolks and 350 g sugar until frothy. Beat the egg whites with 40 g sugar until stiff. Mix the egg yolk mixture and the chocolate butter mixture together, then add the flour and finally the egg whites.
Line a baking sheet with parchment paper and pour in the batter. Grate the rest of the chocolate and sprinkle over it. Bake in a preheated oven at 180 ° C for 20 minutes. Serve with vanilla ice cream.