Preheat oven to 300 °F (140 °C). Line two heavy large baking sheets with foil or parchment paper.
Sift 1/4 cup sugar, cocoa and salt into small bowl.
Beat egg whites and cream of tartar in large bowl until soft peaks begin to form. Add remaining 1/4 cup sugar 1/2 tablespoon at a time and beat until medium-firm peaks form. Add cocoa mixture 1 tablespoon at a time and beat until meringue is stiff and glossy.
Drop meringue onto prepared baking sheets by rounded teaspoonfuls, spacing 1" apart. For chewy soft cookies, bake meringues 25 minutes. For drier, crisper cookies, bake meringues 40 minutes.
Cool cookies on baking sheets. Sift powdered sugar over cookies.