Dissolve the sugar and yeast in the warm milk (or in a mix of warm milk and water). Add the whole egg, the extra yolk, salt and melted butter, then stir to combine. Add the sifted flour and knead until the dough is smooth and elastic, either by hand or with a stand mixer fitted with a dough hook.
Shape the dough into a ball, transfer it to a lightly oiled bowl and cover with a kitchen towel. Let it rise in a warm spot for 1–1.5 hours, until puffy and doubled in size.
Divide the dough into 10 equal pieces (about 80 g each). Shape each piece into a round bun, gently flatten them and let them proof for another 20–30 minutes.
For the filling, blend the farmer’s cheese with the egg yolk, sugar and vanilla sugar (or vanilla extract) until the mixture is smooth and creamy.
Grease the bottom of a glass with a little oil and use it to press deep wells in the center of each bun. The base of the glass should be about 2 1/2 inches (6 cm) wide. Press almost to the bottom, then fill each well with the cheese filling. Leave the buns on the counter for about 20 minutes.
Place a few blueberries on top of the cheese filling so they bake together with the buns, or keep them fresh and add after baking if you prefer. Brush the dough around the filling with an egg wash made from the yolk beaten with 1 tablespoon of milk.
Sprinkle the buns with streusel. To make the streusel, mix the sugar, flour and cold butter, then rub everything together with your fingers until you get coarse crumbs.
Bake the buns in a preheated oven at 350 °F for about 18–20 minutes, until they are puffed and evenly golden on top.