Mix filling together, set aside.
Combine flour, icing sugar and baking powder. Cut butter into dry mixture. Add eggs, egg yolk and sour cream. Knead the dough for a few minutes. Roll out the dough into 2 rectangles approximately 8"x12". Place one rectangle in a buttered 8x12 inch pan.
Bake for approximately 15 minutes, remove from oven.
Pour filling over baked pastry. Cover with second rectangle of dough and return to oven and bake for 45 minutes. Cool and remove from pan.
Sprinkle with icing sugar.