PREHEAT oven to 400° F (375° F. if using a glass baking dish). Butter and flour a 13 x 9-inch baking dish.
PLACE egg whites and cream of tartar in large mixer bowl. Beat until foamy. While beating, slowly add 1/4 cup sugar; beat until soft peaks form. Sprinkle flour over egg white mixture.
PLACE egg yolks and 1/4 cup sugar in small mixer bowl. Beat until thick and lemon-yellow in color. Beat in vanilla extract and salt, then gently fold mixture into egg white mixture just until blended. Spread evenly into prepared pan.
BAKE for 15 to 16 minutes or until golden brown. Immediately loosen the cake from the pan and turn onto a clean towel. Carefully roll up cake and towel together, starting with long side. Cool completely on wire rack.
BEAT cream in small mixer bowl until stiff peaks form; stir in cocoa mix. Place raspberries, vinegar and 2 tablespoons sugar in small saucepan. Cook over medium heat, stirring occasionally, for 5 to 6 minutes or until mixture has thickened. Carefully unroll cake; remove towel.
SPREAD cocoa cream mixture over cake; reroll cake. Place on serving dish. Refrigerate until ready to serve. Serve with warmed raspberry-balsamic sauce.