Heat the water to a warm state, approximately 30-40°C. The water should not be too hot. Pour the water into a deep bowl in which you will knead the dough. Pour in dry yeast, salt and sugar. Mix carefully and leave for 10-15 minutes in a warm place so that the yeast activates.
Then add milk and one chicken egg to the diluted yeast. Mix. The milk should also be slightly warm or at room temperature.
Sift the flour and add to the liquid.
Knead the dough until it begins to come away from the sides of the bowl and your hands. Form the dough into a ball.
Cover the dough with a towel or cling film and leave in a warm place to rise for 1 hour.
Meanwhile, prepare the filling. Add one chicken egg to the mashed potatoes. Mix the egg into the potatoes very quickly, as the protein may curdle. It is better to cool the mashed potatoes a little beforehand. Salt the filling and mix thoroughly.
When the dough has risen, remove it from the bowl and place it on a floured surface.
Divide the dough into portions and roll into balls. Place the pieces on a baking sheet lined with parchment. Grease the parchment with vegetable oil beforehand. Cover the dough pieces with a towel and leave for about 20 minutes to rise.
Gently flatten each ball of dough with your hands and use a glass to make indentations.
Place mashed potatoes into each cavity.
Brush the sides of the vatrushki with egg yolk. Put the vatrushki in the oven to bake for 30 minutes.
The potato vatrushki are ready. Serve the dish sprinkled with chopped greens. Enjoy your meal!