In a small bowl, combine the yeast with half a cup of warm milk and one tablespoon of sugar. Let the ingredients rest on the counter for 10 minutes.
Either by hand or with a stand mixer, combine the eggs in a large bowl with the remaining milk, salt, sugar, oil, vanilla, and sour cream.
Add the yeast mixture and one cup of flour into the egg mixture. Whisk to combine and cover with a towel. Set aside to rest in a warm place for an hour.
With your hands or a stand mixer, mix in the remaining flour in thirds.
Knead the dough until it becomes elastic (at least 10 minutes).
Let the dough rise for an hour.
With lightly oiled hands, make small balls out of the dough, then flatten them out with your fingers and place a dollop of poppy seed filling inside.
Fold over the sides and pinch the edges together to seal the filling (or the dough?)
Fill a deep skillet with 2-3 cups of canola or vegetable oil and set to medium-low heat. Flipping periodically with a fork, fry the piroshky until they turn golden brown all over.
Lay piroshky on a plate lined with paper towels to soak up excess oil. Serve warm and enjoy!