Spray a 9 by 9-inch or 13 by 9-inch baking dish with non-stick cooking spray. Measure out cereal into a bowl and set aside.
Melt butter in a pot (about 5 quart+) over medium-low heat. Add marshmallows and cook, stirring constantly, just until marshmallows have melted.
Remove from heat, pour in Rice Krispies cereal. Gently fold mixture to evenly coat.
Pour mixture into prepared baking dish. Spray a spatula with non-stick cooking spray and use it to spread and gently press mixture evenly into baking dish.
Let rest at least 30 minutes (for sturdier rice krispie treats wait about 2 hours), then cut into squares.
Store in an airtight container in a single layer or with parchment paper between layers for up to 3 days.
Notes
Also try browned butter rice krispie treats: Brown the butter in the pot then remove pan from heat, add marshmallows and stir. Return to medium-low heat to help melt fully then stir in rice krispies cereal.