Break chocolate into pieces and, stirring, melt together with butter and sugar on a bain-marie. Remove from the heat, let chill, mix in whipped egg yolks and flour.

Whisk egg whites into firm foam. Combine 1/3 of whisked egg whites with the dough, then add another part of egg whites and mix carefully. Put the dough in the greased form and bake 35 minutes at 347 °F (175 °C). Cool. Before serving, sprinkle the cake with powdered sugar and garnish with lemon balm.