Step 1Grind or pulse the hazelnuts with the granulated sugar in a food processor until fine, but with a few chunks scattered throughout.
Step 2Cream the butter and brown sugar with the salt until light and fluffy, then stir in the vanilla and egg yolk. Stir in the hazelnuts and the flour.
Step 3Shape the dough into a rough log, wrap it in wax paper or plastic wrap, then run the dough between your thumb and forefinger to force it into a cylinder about 1 1/2 inches in diameter. Keep it round, or square the sides. Refrigerate the dough until firm, at least 1 hour, preferably longer.
Step 4Heat the oven to 350 degrees.
Step 5Cut the log into 1/3- to 1/4-inch thick slices and place them on 2 baking sheets, leaving at least 2 inches between each piece. Bake until lightly browned on top, about 15 minutes. Remove the crisps to a rack to cool; they'll get crispier.