In a small bowl, combine yeast with half a cup of warm milk and one tablespoon of sugar. Let the ingredients rest on the counter for 10 minutes.
By hand or using a stand mixer, combine eggs with milk, salt, sugar, oil, vanilla, and sour cream.
Add 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.
Using your hands or a stand mixer, mix in the remaining flour in by thirds.
Knead the dough until it turns elastic (at least 10 minutes).
Let dough rise for an hour.
With lightly oiled hands, making small balls out of the dough, flatten out a dough ball with your fingers and place a dollop of poppy seed filling inside.
Fold over the sides and pinch the edges together to seal 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!