Grease a 9x13 inch baking dish with butter or line it with wax paper.

In a large saucepan over medium heat, combine the unsalted butter and granulated sugar. Stir until the butter is melted and the sugar is dissolved, about 2-3 minutes.

Add the mini marshmallows and stir continuously until they are completely melted.

Remove the saucepan from heat and stir in the peanut butter until fully combined.

Gradually add the Rice Krispies cereal to the peanut butter mixture and fold until evenly coated.

Pour the mixture into the prepared baking dish and press it down firmly with a spatula or wax paper.

Allow the treats to cool at room temperature for about 30 minutes.

Once cooled, cut the treats into squares or rectangles.