Beat egg whites until stiff and chill. Slowly heat the butter and chocolate separately in a bain-marie and melt. Beat the egg yolks with the sugar until foamy, then add the butter and chocolate.
Then add the flour and mix well. Gently fold in the cold egg whites.
Pour the batter into a greased tart pan and bake in a preheated oven at 200°C for about 20 minutes. After 10 minutes of baking time, arrange the walnuts nicely on the dough and finish baking the tart.