Melt butter in a large saucepan over medium high heat. Once melted, stir in brown sugar and milk. Bring to a boil and stir for 1 minute.
Remove saucepan from heat and stir in peanut butter and vanilla until smooth. Add confectioners powdered sugar and rice cereal and continue to mix until fully combined.
Spray a 8x8 pan with nonstick spray. Pour fudge mixture into the pan.
Optional: Add 2-3 tablespoons of rice cereal on top of the fudge if you want a crispy top (I recommend this). Push cereal lightly down with a spatula so it sets in the fudge.
Cover and place in refrigerator to chill overnight. Once cooled and set, cut into small squares. Enjoy!