Preheat the oven to 356°F / 180°C. Wash, halve, and pit the plums. Leave the skins on, as they soften during baking and add flavor.
Separate three eggs. Beat the whites until stiff. In another bowl, whisk the yolks with 130 g sugar and vanilla sugar until pale and fluffy.
In a mixing bowl, combine buckwheat flour, almond flour, baking powder, cinnamon, and lemon zest. Stir in the softened or melted butter, then fold in the whipped yolks. Gently add the beaten whites last.
Spread the batter into a well-buttered 8-inch (20 cm) round pan, making sure the sides are greased generously to prevent sticking.
Arrange the plums over the base in two layers: first with the cut side up, then a second layer with the cut side down. Bake for 15 minutes at 356°F / 180°C to set the base before adding the cheese layer.
While the base is baking, prepare the filling. Separate the remaining 4 eggs. Beat the whites with 100 g sugar until stiff peaks form. In a separate bowl, whisk the yolks with the other 100 g sugar and freshly squeezed lemon juice until thick and creamy.
Mix the cottage cheese into the yolk mixture until smooth, then fold in the whipped whites by hand to keep the filling airy.
Pour the cottage cheese mixture evenly over the plums. Bake for 35 minutes at 356°F / 180°C. Cover loosely with parchment if the top darkens too fast.
The cheesecake is done when golden at the edges and set in the middle. Let rest for 10 minutes before loosening the edges and unmolding. Allow to cool to room temperature before slicing and serving.