In a bowl, pile up the flour(s), salt, sugar(s) and the cold grated butter. Rub the butter it between your fingers until you get an even crumbly sand-like consistency. If you’re using vanilla extract instead of vanilla sugar, here’s where you can add it.
Start getting the dough together in the bowl, with your hands. You must not knead the dough, but obtain a nice, bit shiny, homogenous dough that sticks together.
Preheat oven to 200C with ventilation, or 210 without. Weigh pieces of around 10g of dough, roll them into small spheres and place on a baking tray lined with baking paper, with some distance in between.
Using a 1/2 tsp measurement, make small holes in the spherical dough. You can, of course use your thumb for that. Then fill the wholes with jam.
Bake in the preheated oven, on the middle rack, for 10 minutes.
Let them set on the tray for half an hour first, then you can transfer them on a cooling rack. They keep well in an airtight container for a week or so.Enjoy with coffee or tea. Or warm cocoa :)