Beat butter, margarine and sugar in large bowl until light and fluffy. Beat inn yolks and vanilla until well mixed. Stir in flour until blended. Divide dough into thirds. Shape into 3 balls. Wrap in wax paper, refrigerate for 15 minutes.
Preheat oven to 350º. Lightly grease 2 baking sheets.
Using one ball of dough at a time. Roll out on a lightly floured surface to ¼” Cut out with a 3” (large) or 2” (small) star cooker cutter.
Transfer cookies to prepared cookie sheets. Leaving 2 inches between each. Brush tops with slightly beaten egg to glaze. Sprinkle with nonpareils.
Bake 10-12 minutes or until golden brown for small stars and 15-17 minutes for large stars.