1. Preheat oven to 375 degrees. Toast hazelnuts in oven on a rimmed baking sheet until skins split and flesh turns deep golden brown, 10 to 12 minutes.

2. When nuts are cool enough to handle, rub off as much of the skin as will come off easily. Let cool completely.

3. Process nuts in a food processor with 1/4 cup sugar until very fine.

4. Beat egg whites with the salt in the bowl of an electric mixer (or by hand), until soft peaks form. Add remaining 1/2 cup sugar, and beat until egg whites hold stiff (but not dry) peaks.

5. Add nut mixture, flour, and vanilla to egg whites; beat until just combined. Refrigerate, covered, until cold, at least 1 hour or up to 1 day.

6. Line a baking sheet with parchment paper or a Silpat baking mat. With cool, dampened hands, mold heaping tablespoons of dough into pyramid shapes, each about 1 1/2 inches wide and 1 1/4 inches high. Transfer to lined sheet.

7. Bake cookies until edges and bottoms begin to brown, 15 to 18 minutes.