Grease a 9x13-inch pan with butter or non-stick spray, then set aside.
In a large saucepan, melt the butter over medium heat until it bubbles and releases a rich, buttery aroma—about 2 minutes.
Add the mini marshmallows to the melted butter. Stir constantly with a spatula or wooden spoon until they melt completely and the mixture becomes smooth and glossy, about 3-4 minutes.
Remove the saucepan from heat. Quickly pour in the crispy rice cereal and fold gently with the spatula until the cereal is evenly coated in the marshmallow mixture.
Transfer the sticky mixture into the prepared pan. Press it down firmly with buttered wax paper or a spatula to create an even layer. Flatten the top and smooth the surface.
Allow the treats to sit at room temperature for at least 30 minutes, until fully set and firm.
Once cooled, cut into squares or fun shapes with a sharp knife or cookie cutters. Serve immediately or store in an airtight container for later.
