In a large pot over medium heat, melt the butter. Once melted, add in the marshmallows and stir until fully melted and smooth, about 5-7 minutes.

Add the cinnamon and mix well. Gradually fold in the Rice Krispies, ensuring that the cereal is fully coated with the marshmallow mixture.

Transfer the mixture into a greased 9x13-inch pan. Use a spatula or your hands to press it down gently, making sure it’s even across the top.

Let it cool for at least 30 minutes before cutting into squares, sprinkling sugar on top if desired.