Grease the baking dish with non-stick spray or line it with parchment paper for easy removal.
Place a large saucepan over medium heat, add the butter, and stir until melted with a glossy, smooth consistency, and you smell a rich buttery aroma—about 2 minutes.
4 tablespoons unsalted butter
Add the marshmallows to the melted butter, stirring continuously until completely melted and the mixture is smooth and glossy. You should see the marshmallows melting and the mixture thickening after about 3 minutes under gentle heat.
4 tablespoons unsalted butter
Remove the saucepan from heat, then immediately pour in the puffed rice cereal. Stir briskly with a rubber spatula until evenly coated and the mixture begins to stick together when pressed, about 1-2 minutes.
4 tablespoons unsalted butter
Transfer the mixture into the prepared baking dish. Using a spatula or lightly greased hands, press the mixture firmly into an even layer. The surface should be smooth and compact, with a slight sheen from the melted marshmallows; press for about 1-2 minutes to ensure it binds together.
Allow the treats to cool at room temperature for at least 30 minutes until set and firm. Once cooled, cut into squares using a sharp knife. The final texture should be chewy yet crisp on the outside.