Grease the bottom and sides of an 8x8 inch baking pan with butter, then line it with parchment paper for easy removal.
In a large medium saucepan, melt the 3 tablespoons of butter over medium heat, stirring occasionally, until it is fully melted and begins to foam with a slightly toasted aroma, about 2 minutes.
3 tablespoons unsalted butter
Add the mini marshmallows to the melted butter in the saucepan. Stir continuously with a rubber spatula until the marshmallows are completely melted and the mixture is smooth, about 4-5 minutes. You should see the mixture become glossy and smooth, with a slight bubbling around the edges.
3 tablespoons unsalted butter
Remove the saucepan from heat, then immediately pour in the rice cereal. Using the rubber spatula, gently fold the cereal into the marshmallow mixture until evenly coated, ensuring no cereal is left dry or clumped.
3 tablespoons unsalted butter
Transfer the mixture into the prepared pan. Press it evenly into the pan using the back of the spatula or wax paper, creating a compact and level layer. Press gently to avoid crushing the cereal, but ensure it's firmly packed.
Allow the treats to cool at room temperature for at least 20-30 minutes until set and firm. Cut into squares and serve once completely cooled and solidified with crunchy edges and chewy centers.
