*Vanilla sugar is available in the baking section of most supermarkets, in bulk food stores and baking supply stores.
1. Beat butter with sugar in a large mixing bowl. Beat in yolks and vanilla. Gradually beat in flour. Turn out onto counter and knead as you would bread until well combined. Form into a ball, cover with a bowl and let stand at room temperature for 2 hours.
2. Preheat oven to 190ºC . To form the crescents, 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 cookie sheets about 3 cm apart. Bake on centre sheet of preheated oven for about 10 to dozen minutes. Cookies should be white in colour. To prevent breaking, cool on cookie sheets for 5 minutes. Then, while cookies are still warm, carefully roll in vanilla sugar. Repeat with remaining cookies. Cool cookies completely.
3. Meanwhile, melt chocolate in a double boiler set over simmering water. When cookies are cool, dip tips of each cookie in melted chocolate, then set on waxed paper to dry.