Preheat oven to 350°F. Grease a 13- × 9-inch pan.

In a large bowl, using electric mixer on low speed, combine cake mix, water, oil, peanut butter, and eggs. Stir in chocolate pieces. Pour into prepared pan.

Bake 30 to 35 minutes or until a toothpick inserted in the center of the cake comes out clean.

Pour marshmallows over top of cake. Return to oven and bake 2 to 3 minutes or until marshmallows are somewhat melted.

In a microwave-safe bowl melt fudge frosting in microwave.

Drizzle frosting over melted marshmallows. Sprinkle pecan pieces over frosting.

Allow cake to cool completely before serving. Makes 10 to 12 servings.