PREHEAT oven to 300°F. Line 2 baking sheets with foil.

SIFT together cocoa, salt, and ¼ cup of the sugar in small bowl.

BEAT egg whites and cream of tartar in large bowl with an electric mixer at medium-low speed, until soft peaks form. Beat in remaining ¼ cup sugar, ½ tabbespoon at a time, until meringue is glossy and stiff peaks form. Fold in cocoa mixture and vanilla extract.

DROP by rounded teaspoonfuls about 1" apart onto prepared baking sheets. Bake 25 minutes for soft chewy cookies or 40 minutes for crisp ones. Dust cooled cookies with confectioners' sugar.