1. Preheat oven to 325°.
2. Combine first 3 ingredients in a food processor; process until finely ground. Combine hazelnut mixture, 1/4 cup powdered sugar, and cornstarch in a medium bowl, stirring with a whisk.
3. Place egg whites, cream of tartar, and salt in a large bowl; beat with a mixer at high speed until soft peaks form. Add granulated sugar, 1 tablespoon at a time, beating at high speed until stiff peaks form. Gently fold hazelnut mixture and vanilla into egg mixture.
4. Cover two baking sheets with parchment paper. Spoon egg white mixture into a pastry bag fitted with a 1/2-inch round tip. Pipe 48 (1 1/2-inch-round) mounds onto prepared baking sheets. Bake at 325° for 25 minutes or until meringues are done and dry to the touch. (Meringues are done when surface is dry and meringues can be removed from paper without sticking to fingers.) Turn oven off; partially open oven door. Cool meringues in oven 30 minutes. Remove from oven; carefully remove meringues from paper.