4 teaspoons baking powder
1 cup white sugar
2 teaspoons vanilla sugar
2 pinches salt
1 1/3 cups unsalted butter, room temperature and cut into chunks
2 eggs
2 1/4 pounds Italian prune plums, halved and pitted
2 tablespoons white sugar
1 teaspoon ground cinnamon
2 tablespoons unsalted butter, room temperature and cut into chunks
1 tablespoon sliced almonds (optional)
Combine flour and baking powder in a large bowl. Mix in 1 cup sugar, vanilla sugar, and salt. Add 1 1/3 cups butter and eggs. Knead everything into a smooth dough. Shape dough into a ball, press flat, cover, and chill in the refrigerator 1 hour to overnight. Preheat the oven to 350 degrees F (175 degrees C). Grease a baking sheet with butter. Cut halved plums in half again, without cutting all the way through, so they fan out. Roll pastry dough out on a lightly floured work surface. Line the bottom and sides of the prepared baking sheet with dough. Mix 2 tablespoons sugar and cinnamon together in a bowl. Arrange plums side by side on the pastry dough; very small plums should overlap. Dot plums with 2 tablespoons butter and sprinkle with cinnamon sugar and almonds. Bake in the preheated oven until pastry crust is lightly browned, 30 to 40 minutes. Remove from oven. Allow plum cake to cool, then cut into squares. Serve lukewarm or at room temperature.