Toasted ground hazelnuts make the cookie dough extra rich. The crescents are shaped by hand and drizzled with both semisweet and white chocolate for a truly exquisite cookie.
In large bowl, beat butter, both sugars, 1 tablespoon water, vanilla, nutmeg and salt at low speed for 2 minutes. Scrape down bowl. Beat at medium-low until fluffy, 2 minutes. Beat in yolk, then ground hazelnuts. At lowest speed, beat in flour, just until blended. Dough should be stiff, but still soft; add a little more flour if needed.
Turn out onto floured surface. Divide in half. Shape each into a block, 1-inch thick. Wrap in plastic. Refrigerate 1 hour.
Heat oven to 350°F. Break off 2 tablespoons of dough. Warm slightly with hands and roll to a 3-inch log. Curve ends to form a crescent. Space 1 inch apart on ungreased baking sheets. Repeat for a total of 48.
Bake at 350°F for 13 to 15 minutes, until uniformly tan. Cool on wire racks.
Set racks over sheets of parchment or waxed paper. Place melted semisweet and white chocolates in two resealable plastic bags. Snip off a small corner of each. Drizzle over cookies; let harden.