1. Move oven rack to lowest position. Mix, bake and cool cake as directed on package.
2. Sprinkle gelatin over water in small microwave-safe bowl; let stand 2 minutes to soften gelatin. Microwave at HIGH (100%) 40 seconds; stir thoroughly. Let stand 2 minutes or until gelatin is completely dissolved; cool slightly. Stir together yogurt, strawberry syrup and gelatin mixture in medium bowl until smooth; refrigerate until mixture mounds slightly when dropped from spoon, about 15 to 20 minutes.
3. Place cake, rounded side down, on cutting board. Using serrated knife, cut 1-inch layer from top of cake; lift off in one piece. Using serrated knife, cut around cake 1-inch from center hole and 1-inch from outer edge, leaving cake walls and cake base 1-1/4-inches thick. Using fork, carefully remove cake in cavity without breaking through sides or bottom of cake. Place hollowed-out cake on serving plate. Spoon strawberry syrup mixture into cavity. Cover with reserved cake top. Cover; refrigerate while preparing CHOCOLATE SYRUP WHIPPED TOPPING. Spread topping evenly over top and sides of cake.
4. Refrigerate 4 hours or until strawberry syrup center is set. Garnish as desired. Serve cold. Cover; refrigerate leftover cake. Makes 12 servings.
CHOCOLATE SYRUP WHIPPED TOPPING: Stir together 2 cups frozen light whipped topping, thawed and 1/4 cup HERSHEY'S Syrup in medium bowl.