In a mixing bowl, dissolve yeast in water; let stand for 5 minutes. Add sour cream, egg, sugar, butter, salt and 2 cups flour; beat until smooth. Add enough remaining flour to form a soft dough. Turn onto a floured surface; knead until smooth and elastic, abut 6-8 minutes. Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 1 hour. Punch dough down. Reserve 1 cup dough. Divide remaining dough in half. Roll each portion into a 9-in. circle; place in greased 9-in. round baking pans. Combine filling ingredients; spread over dough to within 1/2 in. of edge. Roll out reserved dough to 1/4-in. thickness; cut into 1/2-in. strips. Make a lattice top over filling. Cover and let rise until doubled, about 45 minutes. Bake at 375 degrees F for 15 minutes. Cover top with foil; bake 20 minutes longer or until browned.