Put the flour, butter, powdered sugar, vanilla sugar, almond meal, egg yolk, and salt into the bowl of a stand mixer fitted with the paddle attachment. Beat together, starting on low so the flour doesn’t fly everywhere, until a smooth dough develops.
Divide the dough into 3 equal portions and roll each portion into a 1-inch-thick cylinder. Wrap the rolls individually in plastic wrap and refrigerate for at least 1 hour and up to 24 hours.
When you’re ready to bake, heat the oven to 350 degrees F and line two baking sheets with parchment paper.
Remove one roll from the refrigerator and cut it into 1/2-inch slices. Roll out each slice until it’s about 2-inches longer and the ends are tapered but not pointy (this may take a bit more pressing and shaping rather than simply rolling because the dough can be crumbly). Form each into a crescent and place on one of the prepared baking sheets. Repeat with the remaining dough until the baking sheet is filled. The cookies should have about 1 inch of space between them on all sides.
Bake one pan at a time in the center of the oven for 10 to 15 minutes, or until the cookies are just starting to lightly brown at the edges. Repeat with the remaining pan of cookies.
To make the topping, in a wide, shallow bowl combine the powdered sugar and the vanilla sugar. Remove the cookies from the oven, let cool for only a minute or two, and then very carefully dredge them completely in the sugar. Place on a rack to finish cooling. The cooled cookies can be stored in an airtight container for up to 3 weeks.