Dissolve the sugar and yeast in the warm milk (or in a mix of warm milk and water). Add the whole egg, the extra egg yolk, salt and melted butter. Stir to combine, then add the sifted flour. Knead the dough until smooth and elastic. You can knead by hand or with a stand mixer fitted with a dough hook.
Shape the dough into a ball and place it into a lightly oiled bowl. Cover with a clean kitchen towel and let it rise in a warm place for 1–1.5 hours, or until doubled in volume.
Divide the dough into 10 equal pieces (each piece will be about 80 g). Shape each piece into a smooth bun, gently flatten them and let them proof on the tray for another 20–30 minutes.
Prepare the filling: blend the cottage cheese or farmer’s cheese together with the egg yolk, sugar and vanilla sugar until you get a smooth, creamy mixture.
Using the bottom of a glass greased with a little oil, press deep wells into the center of each bun. The base of the glass should be about 6 cm in diameter. Press firmly, almost all the way down, then fill the wells with the cheese filling. Leave the buns on the counter to rest for 20 minutes.
Place a few blueberries on top of the filling so they bake together with the buns, or, if you prefer, you can add the berries only after baking. Brush the exposed dough with an egg wash made from the egg yolk beaten with a tablespoon of milk.
Sprinkle the buns with streusel. To make the streusel, rub together 30 g sugar, 30 g flour and 15 g butter with your fingertips until you get coarse crumbs.
Bake the buns in a preheated oven at 180 °C (350 °F) until golden, about 18–20 minutes. Let them cool slightly before serving.