Cream the butter and sugar. Add egg yolk and vanilla and beat thoroughly. Combine flour and salt. Add to the butter mixture and blend until the dough is smooth. On a very lightly floured surface, shape the dough into cylinders about 1 to 2 inches in diameter. Wrap in waxed paper or plastic wrap and chill for at least an hour. Preheat oven to 350 degrees. Brush refrigerated logs with beaten egg and roll in crystalline sugar. With a sharp knife, slice the dough into rounds about 1/3-inch thick. Place 1-inch apart on a parchment covered baking sheet. Bake for about 10 minutes or until the cookies are slightly colored around the edges. Yield: approximately 4 dozen Sables