1. Drain ricotta overnight: line a colander with cheesecloth and place it over a large bowl. (This step is optional, you can add another tablespoon of semolina to thicken the filling).
2. In a bowl, stir together softened butter and sugar. Add sour cream and eggs, mix until well incorporated.
3. Combine flour and baking powder, gradually add to the mixture, mixing with a spoon, then with your hands. The dough should be soft and smooth. Form dough into a log.
4. Prepare the filling. In a separate bowl, combine ricotta, sugar, semolina, vanilla extract and egg yolk.
5. Divide the log of dough into 9 equal portions (original recipe).
6. With the palm of your hand, press each portion of the dough to form a 5-inch circles.
7. Preheat oven to 375 degrees F (190 degrees C). Cover a baking sheet with parchment paper.
8. Place filling (about 1 1/2 tablespoon) onto one half of each circle.
9. Lift the other half of each circle to cover the filling, pressing down very lightly.
10. Arrange sochniki on the baking sheet. Lightly beat egg white with a fork. Brush sochniki with egg white.
11. Bake in the preheated oven for 25-30 minutes, until golden brown. Enjoy!