Cream together the butter with the sugar in a large mixing bowl. Beat in egg yolks and vanilla. Gradually beat in flour.
Turn into a ball, cover and let stand at room temperature for 2 hours.
Preheat oven to 190 degrees C.
To form the biscuits, roll about 1 tablespoon of the dough in the palms of your hands. Then continue rolling to form a rope about 8 cm long and 1 cm thick. Bend rope to form crescent. Repeat with remaining dough, placing on ungreased baking trays about 3 cm apart.
Bake for about 10 to 12 minutes. Biscuits should be white in colour. Let cool on baking trays for 5 minutes to prevent breaking. Then, while biscuits are still warm, carefully roll in vanilla sugar. Repeat with remaining biscuits. Cool biscuits completely.
Melt chocolate in a double boiler set over low heat. When biscuits are cool, dip tips of each biscuit in melted chocolate then set on waxed paper to dry.