For the rhubarb filling: Combine rhubarb and sugar in a large bowl. Mix to combine and set aside to allow the rhubarb to release some juice, about 1 hour.
For the cake: Beat butter and sugar together in a large bowl until light and fluffy. Add flour, wine, baking powder, and egg; mix together to form a dough. Wrap dough in plastic wrap and place in the refrigerator for 5 to 10 minutes.
Meanwhile, preheat the oven to 350 degrees F (175 degrees C). Grease a 9-inch springform cake pan.
Roll dough out on a lightly floured surface into a circle slightly larger than the diameter of the cake pan. Transfer to the prepared cake pan, letting the dough come up the sides by about 1 inch.
Drain rhubarb and evenly scatter over the dough; bake in the preheated oven for 45 minutes.
Meanwhile, make the custard filling: beat almond flour, 3/4 cup plus 2 tablespoons crème fraîche, sugar, eggs, vanilla sugar, and cinnamon together until smooth.
Remove cake from oven after 45 minutes; top with almond custard mixture and use a spatula to spread it evenly to the sides of the pan. Return cake to the oven and bake until golden brown on top, about 30 minutes more.
Allow the cake to cool in the pan before slicing.
