In a large saucepan, melt the butter and marshmallows over medium heat, stirring frequently.
When marshmallows are melted, remove from heat, and stir in the vanilla extract, 1 ¼ cups of chocolate peanut butter and the cereal until completely blended.
Spread mixture into a greased 9 x 13 pan, and set aside.
In a small bowl, melt the chocolate chips in the microwave, and stir in the remaining chocolate peanut butter until thoroughly blended.
Spread the mixture over the chocolate cereal bars, and refrigerate until frosting sets and bars are firm enough to cut.