In a large bowl, cream butter and sugar until light and fluffy. Beat in egg yolks and vanilla. In a small bowl, combine flour and salt; gradually beat into creamed mixture, mixing well (dough will be very stiff).
On a well-floured surface, roll out dough to 1/8-in. thickness. With a sharp knife or pastry wheel, cut dough into 2-1/2-in. squares, rectangles or diamonds. Place 1 in. apart on ungreased baking sheets. Sprinkle with colored sugar.
Bake at 375° until lightly browned, 7-8 minutes. (Watch carefully as cookies will brown quickly.) Remove to wire racks to cool. Store in airtight containers.