Defrost the cranberries. Put flour, butter, sugar, vanilla sugar, 1 pinch of salt and egg yolk in a bowl. First knead into crumbles with the dough hooks of the hand mixer, then briefly with your hands.
Wash, peel and quarter the apples and remove the core. Cut quarter into small pieces. Mix apples and cranberries, fold in grated coconut and apple sauce and mix again. Grease a square springform pan (24 x 24 cm).
Spread about half of the crumbles in it and press to a flat bottom.
Spread the apple-cranberry mixture on the base, sprinkle the remaining crumbles over it. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 35-40 minutes. Remove from the oven and let it cool down on a cake rack.
Melt the couverture over a warm water bath. Cut the cake into longish pieces. Sprinkle with the couverture and serve.