Chocolate Apple Upside-down Cake
Peel, halve and core apples and cut into thin slices. Melt butter, stir in apples and sugar and simmer until syrupy.
Break dark chocolate into pieces and melt over a hot-water bath. Remove from heat, add butter and sugar and stir until melted. Separate eggs, stir yolks and flour into chocolate mixture and mix well. Beat egg whites until stiff, carefully fold into chocolate mixture and stir in hazelnuts.
Line rectangular baking pan with parchment paper.
Arrange cooled apple slices in prepared pan and smoothly and evenly spread batter over apples.
Bake cake in oven preheated to 180°C (approximately 350°F) until cake tests done with a toothpick, 40-50 minutes.
Let cake cool in pan, then invert pan to remove cake. Cut cake into rectangular pieces to serve.