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HEAT oven to 350ºF.
ADD 1 Tbsp. dry gelatin mix to frosting; stir gently until blended. Refrigerate until ready to use. Add boiling water to remaining gelatin mix in medium bowl; stir 2 min. until completely dissolved. Pour into 8x4-inch loaf pan sprayed with cooking spray. Refrigerate 1 hour or until firm.
MEANWHILE, beat cake mix, dry pudding mix, eggs, cold water and peanut butter with mixer until blended. (Batter will be thick.) Spoon into 24 paper-lined muffin cups. Bake 18 to 21 min. or until toothpick inserted in centers comes out clean. Cool completely.
USE paring knife to cut 1-1/4-inch wide piece from top of each cupcake, being careful to not cut through to bottom of cake; set removed centers aside. Cut gelatin into 24 cubes; place in cupcakes. Cover with removed cupcake centers, pressing gently into cupcakes to secure. Spread tops with flavored frosting.