1. Heat oven to 300°F. Cover cookie sheet with parchment paper or foil.
2. Beat egg whites and cream of tartar in large bowl at high speed of mixer until soft peaks form. Gradually add sugar and vanilla, beating well after each addition until stiff peaks hold, sugar is dissolved and mixture is glossy. Sift cocoa onto egg white mixture; gently fold just until combined. Fold in chocolate chips. Drop mixture by heaping tablespoons onto prepared cookie sheet.
3. Bake 35 to 45 minutes or just until dry. Cool slightly; peel paper from cookies. Cool completely on wire rack. Store, covered, at room temperature. 30 cookies.