Dissolve yeast in the lukewarm milk or water.
In the bowl of an electric mixer (use the paddle), combine the butter, salt, vanilla, lemon zest and sugar. Mix on low for 3 minutes.
Add the flour and sour cream alternately.
Add 2 eggs and beat for 3 to 4 minutes, or until dough pulls away from the sides of the
bowl.
Divide the dough into 28 equal pieces.
Roll them into balls then place on cookie sheet.
Cover then refrigerate for 15 minutes.
Roll each ball into a circle on a floured board.
Place 1 heaping teaspoon to tablespoon filling in the center of each circle. Pinch up the sides to form a triangle.
Place the hamantaschen on a greased or parchment paper-covered cookie sheet. Brush the top with the beaten egg.
Preheat oven to 375° F.
While the oven is preheating, let the hamantaschen rise for 30 minutes.
Bake about 18 minutes, or just until golden brown.