In a LARGE microwave-safe bowl, add butter, peanut butter, and 1 (10 oz.) package of mini marshmallows. Reserve the other 5 oz. of marshmallows for later.
Microwave for 45 seconds, stir, then continue heating in 15-20-second increments until melted and gooey. Stir.
Stir in the cereal and the rest of the mini marshmallows. Press the mixture into a buttered 9" x13" pan. Pack down slightly.
Melt chocolate chips and butterscotch chips in a double boiler or microwave.
Spread chocolate mixture over Scotcheroo base and refrigerate until set, about 30 minutes.
Cut into bars. Store the treats in an airtight container or wrap them individually.
Video
Notes
Stove-top directions: heat butter, marshmallows, and peanut butter in a large pot over medium-low heat until melted and smooth. Stir in cereal, mini marshmallows, and peanut butter chips using a wooden spoon. Prepare the rest the same as above.