As already explained for quadruple chocolate muffins, chop chocolate and put it in the freezer at least half an hour before starting to prepare the cake (but you can also do it the day before), so that it won't settle on the bottom of the cake. Preheat oven to 356° F (180° C). Melt the butter and let it to cool down. Beat the egg whites and sugar with a whisk until mixture is puffed and shiny: it will take 5 to 10 minutes, but they must be stiff for the cake to succeed .. lifting the whisk the mixture should do "peaks" (I had no idea what this meant, before actually doing it .. but if you beat the egg whites properly, you'll understand immediately what I'm talking about: it's a challenge!). Now mix egg whites (being careful not to deflate them) with chopped hazelnuts, chopped chocolate, flour and butter, so that you obtain an homogeneous mixture. Grease and flour a plum cake mold, pour in the mixture and bake it in preheated oven for 30 to 40 minutes: check the internal cooking with a toothpick, but open the oven only after 30 minutes, otherwise the plum cake could deflate. As you can see, this cake does not contain baking powder, so it's important to let the cake "inflate" naturally! Another warning: don't expect the cake to become brown because it will never happen (unless you burn it) … this cake remains quite pale, but still golden brown. If you like, sprinkle the cake with powdered sugar.