Preheat oven to 300°F. Line two baking sheets with foil.
Meanwhile, in a small bowl, sift together cocoa, salt, and ¼ cup sugar. Set aside.
In a large bowl, beat egg whites and cream of tartar with electric mixer at medium-low speed, until soft peaks form. Beat in remaining ¼ cup sugar, teaspoonful at a time, until meringue is glossy and stiff peaks form. Fold in cocoa mixture and vanilla extract.
Drop by rounded teaspoonfuls about 1-inch apart onto baking sheets. Bake 25 minutes for soft, chewy cookies, or 40 minutes for crisp cookies. Let cool before dusting cookies with confectioners' sugar.