LINE 2-1/2-qt. bowl with plastic wrap; spray with cooking spray. Cut cake into 12 slices. Trim crusts; discard. Line bowl with cake slices, cutting to fit, if necessary.
STIR boiling water into dry gelatin mix in bowl 2 min. until completely dissolved. Stir in cold water. Remove 1-1/2 cups of the gelatin; place in large bowl. Refrigerate until slightly thickened. Meanwhile, pour remaining gelatin over cake in bowl.
STIR whipped topping into thickened gelatin. Reserve 3 raspberries for garnish. Gently stir in remaining mixed berries. Spoon over cake in bowl. Refrigerate 4 hours or overnight until firm.
INVERT bowl onto moistened plate; shake to loosen gelatin. Gently remove bowl. Remove plastic wrap. Garnish with reserved 3 raspberries and fresh mint, if desired.