Mix yeast in water and stir in 3 tablespoons flour. Let stand for one hour. Beat eggs, sugar, butter and shortening until creamy. Add to yeast mixture. Blend in sour cream and vanilla. Slowly add flour to make a soft dough, kneading by hand. Make into a large ball and refrigerate overnight. Take a small portion of dough (keep the rest refrigerated) and roll out very thin on a powdered sugar pastry cloth. (Do not use flour.) Cut into 5 cm squares. Put 1 teaspoon of filling in center of square and pinch two opposite corners together. Place on ungreased cookie sheet. Bake in 350 oven for 10 to dozen minutes. When cool, sprinkle with powdered sugar.