BEAT butter, cream cheese and sugar in large bowl with electric mixer on medium speed until well blended. Blend in egg yolks, vanilla and peel; set aside. Mix flour, baking powder and salt. Gradually add to butter mixture, mixing well after each addition. Cover and refrigerate 1 hour.
PREHEAT oven to 350°F. Place dough on lightly floured surface; roll out to 1/4-inch thickness. Cut into circles with 2-inch round cookie cutter. Place on ungreased baking sheets; brush with beaten egg whites. Indent centers; fill with preserves.
BAKE 15 minutes or until edges are lightly browned.