For the dough: Dissolve the yeast in warm (38-40°C) milk, add sugar, egg and whip with a wire whisk until smooth. Pour a little flour and salt. Now it’s better to stir with a mixer using a hook-fitting. Put in butter and continue adding flour until the dough stops to be sticky but remains soft enough. Cover with a film and towel and leave in a warm place for 1,5-2 hours. For the filling: whip the sour cream, eggs and sugar with a wire whisk, add a little vanilla extract. Preheat the oven to 160°C. A cast iron piepan or a deep (about 4 cm) frying pan (26 cm in diameter) are well suited for the smetannik. Lubricate the pan with vegetable oil. Shape a ball from the ready dough, roll it out into a thin flat cake 10 cm in diameter larger than the pan. Lay the dough on the pan, flatten it out with your hands so that it fits repeating the shape and hangs down along the edges for 2-3 cm. Pour the filling onto the dough (it's just fine to be liquid). Carefully cut the edges and lay over the filling or cut figurately and decorate. Place the pan in the oven and bake for 30-40 minutes without opening the door. The filling will turn bright yellow: it should not be brown or very toasty. It can puff up here and there: that’s okay. The dough should be golden brown. When it becomes clear that the pie is ready, turn off the oven and leave the smetannik inside for 20 minutes. Then take it out and cool.