In large mixing bowl, combine cake mix, ½ cup softened butter and egg until well combined. Press into bottom of a greased 15x10x1-inch jelly-roll pan. Bake in a preheated 350°F oven 12-15 minutes.
Remove from oven and sprinkle with marshmallows. Return to oven and continue baking until marshmallows begin to puff (about 1-2 minutes). Remove from oven and allow to cool.
Meanwhile, in large saucepan combine peanut butter chips, corn syrup, remaining ¼ cup butter and vanilla over medium heat. Cook until chips and butter are melted and mixture is smooth, stirring constantly.
Remove from heat and stir in cereal and peanuts. Carefully pour over marshmallows and spread to cover. Cool completely in refrigerator; cut into bars. Store in an airtight container in refrigerator.
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