For the lemon buttermilk tart, prepare a shortcrust pastry from the ingredients: Place flour on a baking board. Press a well in the center. Add egg, sugar and vanilla sugar. Spread cold fat in pieces on top.
Quickly knead all the ingredients together with a pinch of salt to form a dough (with cold hands). Wrap in foil and let rest in the refrigerator for half an hour.
Preheat the oven to 170 °C.
For the filling, grate the zest from the lemons. Whisk the buttermilk with the egg yolks and stir in the lemon zest, vanilla sugar and flour. Season with a pinch of salt and cardamom to taste.
Grease a springform pan (26 cm ø). Roll out the short pastry on a lightly floured work surface and line the pan with it. Pull up the edges a little.
Spread the filling on the dough and bake the lemon buttermilk tart for about half an hour until the filling is lightly browned.