Preheat oven to 375°F.
In a small bowl, dissolve the yeast in the lukewarm liquid.
In the bowl of an electric mixer(use the paddle), combine the butter, salt, vanilla, zest, and sugar. Add yeast and beat on low for 3 minutes then slowly add the flour and sour cream, alternating them.
Add 2 of the eggs and beat for 3- 4 minutes. The dough should pull away from the sides of the bowl.
Divide the dough into 36 equal pieces. Roll them into balls then place them on a parchment paper covered cookie sheet. Cover and refrigerate for 15 minutes. Roll each ball out into a 4” square.
Place 1 Tablespoon of the filling in the center of each circle. Moisten the sides of the square with water. Fold one corner over to reach the opposite corner. Press firmly to seal and shape into a triangle shape with your fingers. Place a small slit on the top of each one to allow the steam to escape.
Place the hamantaschen on a greased or parchment paper covered cookie sheet. Brush the top with the beaten egg.
Let the hamantaschen rise for 30 minutes.
Bake for 16 to 20 minutes, unti lgolden brown.