The dough for these butter cookies is rolled in different colors of crystal sugar. Set them out at a party and let guests pick their favorite color.
In large bowl, with mixer on high, beat butter and sugar until light and fluffy, about 3 minutes. add egg yolks, egg, vanilla, and salt, and beat well to combine. With mixer on low, gradually beat in flour. Divide dough into 6 equal pieces, roll each piece into an 8-inch log, cover with plastic wrap, and refrigerate for 3 hours.
Heat oven to 350°F. Working with 1 roll at a time, unwrap dough and roll in 1 color of crystal sugar. Cut logs into 1/4-inch thick slices, and place on ungreased baking sheet 1/2 inch apart. Bake 12 to 16 minutes until golden. Let cool 5 minutes on baking sheet, transfer to wire rack, and let cool completely. Repeat with remaining dough, using a different color for each roll.