Place egg whites in a small bowl; let stand at room temperature for 30 minutes. Beat egg whites until foamy. Sprinkle with cream of tartar and salt; beat until soft peaks form. Gradually add sugar and vanilla, beating until stiff peaks form, about 5-8 minutes. If desired, divide batter in half and fold in red and green food coloring. , Drop by rounded tablespoonfuls 1-1/2-in. apart onto lightly greased baking sheets. Press a chocolate kiss into the center of each cookie and cover it with meringue, using a knife. , Bake at 275° for 30-35 minutes or until firm to the touch. Immediately remove to a wire rack to cool. If desired, sprinkle with cocoa. Store in an airtight container.