Place the marshmallows, butter, and peanut butter in a large microwave safe bowl, and microwave in 30 second increments. Stir after each 30 second increment to coat the marshmallows in the butter and peanut butter. Repeat until marshmallows are poofy and melted. This will take approximately 3-4 rounds (or 1.5-2 minutes) to complete, depending on your microwave.*
If you are adding the option vanilla, stir in after the marshmallows have been melted.
Stir in 5 cups of puffed rice cereal until evenly distributed.
Gently press the cereal and marshmallow mixture into a 9x13 pan with a clean rubber scraper.
Let your Rice Krispie peanut butter treats set up for 20-30 minutes before cutting.
Serve and enjoy!
Store leftover bars in an air-tight container on the countertop for up to 3 days (I will be impressed if they last that long!)
Notes
*You can also melt the marshmallows, butter, and peanut butter on the stovetop if you prefer. Start by melting the butter in a large pot over medium heat. Then add the marshmallows and peanut butter and stir continuously until melted.