In 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 30-35 minutes, or until toothpick inserted in center of cake comes out clean.
Pour marshmallows over top of cake. Return to oven and bake 2-3 minutes, or until marshmallows are somewhat melted.
In microwave-safe bowl, melt fudge frosting in microwave.
