Wash, pat dry, cut in half and core the grapes. Preheat the oven to 200 ° C. Line a baking sheet with parchment paper.
For the dough, stir the butter and powdered sugar until frothy and slowly beat the eggs in. Sift the flour, mix with the baking powder and stir alternately into the butter mixture with the milk. Mix the ingredients for the crumbles and then work them into crumbles between the palms of your hands.
Spread the batter on the baking sheet. Cover the grape halves evenly, then distribute the crumble evenly over the top.
Turn the baking temperature down to 170 ° C and bake the cake in it for about 50 minutes. Let cool, sprinkle with powdered sugar and serve.