In a small bowl, mix together the sugar and cinnamon. Set aside.

In a large saucepan, melt the butter over low heat. Add the marshmallows and stir continuously until completely melted and smooth.

Remove the saucepan from heat and immediately add the Rice Krispies cereal. Stir gently until the cereal is fully coated with the marshmallow mixture.

Lightly grease a 9×13-inch baking pan or line it with parchment paper. Transfer the mixture to the pan and press it down evenly with a buttered spatula or wax paper.

While the treats are still warm, sprinkle the cinnamon sugar mixture evenly over the top. Press lightly to help it stick. Allow the treats to cool completely before cutting into squares.