Preheat oven to 180 °C top and bottom heat. Grease an ovenproof dish with butter.
Pat the washed grapes dry, cut them in half and remove the seeds.
Beat the egg whites with salt until stiff.
In another bowl, beat egg yolks together with sugar and vanilla sugar until foamy. Add almonds to egg yolk mixture. Fold in egg whites.
Pour almond paste into the pan and place grape halves, cut side down, on top.
Place the almond cake in the preheated oven and bake for 35 minutes.