Prepare 1 (18- to 19.5-oz.) brownie mix, substituting milk for the water in the package directions. Spoon into 12 paper-lined muffin cups; bake at 350ºF. for 25 minutes or until toothpick inserted in center comes out almost clean with some crumbs attached. Cool.
Using 3/4-inch plain pastry tube or small spoon, remove center of cupcakes almost to the bottom. Pipe marshmallow creme into center. Bring 1/4 cup heavy whipping cream to a boil; pour over 4 oz. chopped semisweet chocolate. Stir until smooth. Spread over top of cupcakes. Whisk 1/2 cup powdered sugar and 1 1/2 to 2 teaspoons milk until smooth. Pipe a swirled line over center of cupcakes; let stand until set.
PER SERVING: 320 calories, 15 g total fat (4.5 g saturated fat), 2.5 g protein, 47.5 g carbohydrate, 30 mg cholesterol, 130 mg sodium, 1.5 g fiber