Butter or spray an 8-inch square pan with non-stick spray.
Place your cereal in a large bowl and set aside.
In a medium saucepan, melt the butter over medium-low heat. As the butter melts, it'll start to foam, then turn a clear golden color. Continue to stir the butter, scraping up any browned specks from the bottom of the pan as it starts to brown. It should have a nutty aroma.
Once browned and nutty, turn off the heat and immediately stir in the marshmallows. The heat from the butter should be enough to melt the marshmallows, but if not, put the pan back on low heat until the marshmallows are fully melted and smooth.
Pour the marshmallow mixture over the cereal and stir until incorporated. Quickly spread into your prepared pan. (I like to spray my fingers with cooking spray to press the mixture evenly into the corners and edges of the pan, but a rubber spatula works just as well.)
Melt white chocolate in a small microwave-safe bowl for one minute in the microwave. Stir. Place back in the microwave for another 30 seconds or until no more lumps remain. Use an offset spatula (or butter knife) to spread the chocolate evenly over the Rice Krispies. Allow to set for at least one hour before cutting into squares. Store Rice Krispies Treats covered at room temperature for up to 1 week.