Line a 13x9-inch baking pan with parchment paper or foil.
In a large pot, melt butter over medium heat. Add ¾ of the marshmallows. Stir with a silicone spatula until fully melted, about 5 minutes. Remove from the heat.
Add the cereal and stir until fully combined (mixture will be very sticky). Stir in the remaining marshmallows until evenly dispersed but not fully melted. (This will give little pockets of marshmallows in the treats!)
Pour the mixture into the baking pan. Using the silicone spatula or the butter stick wrapper, press the mixture into an even layer into the pan. Let cool completely, about 1 hour.
Use the parchment to remove the treats from the pan. Lightly grease a sharp knife and cut into squares.
For richer and creamier rice crispy squares, use European butter. European butter has a higher butterfat content than American butter, so as the water evaporates when the butter melts, you’ll be left with more buttery goodness if you use European butter.
Regular marshmallows could be used in place of the mini marshmallows, but you’ll have to keep a close eye on them. They melt much slower.
Avoid overcooking the marshmallows as the marshmallows they will get hard.
Avoid over-stirring the mixture as it will cause the cereal to break and make the squares dense.
When pressing the mixture into the pan, don’t press too hard as you don’t want the squares to be dense and hard.