Cream together the butter and vanilla sugar until combined and pale in color.
Add the flours and mix until just combined.
Cover and refrigerate the dough for at least one hour.
Remove the dough from the refrigerator and preheat the oven to 350F. Remove small pieces of dough and roll into long, thin cylinders of about ½-inch in thickness. Mold the cylinders into a U-shaped horn, and place them on a baking sheet.
Bake the cookies for about 12 minutes. The cookies should not darken noticeably, but should firm up.
Once removed from the oven, allow the cookies to cool for a few minutes, so that they do not fall apart when handled, but not entirely to room temperature. Add the granulated sugar to a bowl, and roll each cookie in the sugar topping.