1. Preheat oven to 250°.
2. Place egg whites in a large bowl; beat with a mixer at high speed until foamy. Add cream of tartar and salt, beating until soft peaks form. Gradually add sugars, 1 tablespoon at a time, beating until stiff peaks form. Add vanilla; beat 1 minute.
3. Cover 2 baking sheets with parchment paper. Spoon 24 (2-inch-round) mounds onto prepared baking sheets. Place in oven; bake at 250° for 1 hour or until dry to touch, rotating pans halfway through cooking. (Meringues are done when surface is dry, and meringues can be removed from paper without sticking to fingers.) Turn oven off. Cool meringues in closed oven 1 hour. Remove from oven; carefully remove meringues from paper.
4. Place chocolate in a medium glass bowl. Microwave at HIGH 1 minute or until almost melted, stirring until smooth. Dip side of each meringue in melted chocolate and chopped hazelnuts.