Step 1: Prepare Your Baking Dish - Start by greasing your 9x13 inch baking dish with a little butter.
Step 2: Mix Sugar and Cinnamon - In a small bowl, combine the sugar and cinnamon. Set it aside for later use.
Step 3: Melt Butter and Marshmallows - In a large saucepan, melt the stick of butter over medium heat. Add the marshmallows and stir until melted.
Step 4: Combine Rice Krispies - Remove from heat and add the Rice Krispies cereal. Stir until coated.
Step 5: Add Cinnamon-Sugar Mixture - Sprinkle half of the cinnamon-sugar mixture over the Rice Krispies and stir to combine.
Step 6: Press Mixture into the Baking Dish - Transfer the mixture to the baking dish and press down firmly.
Step 7: Sprinkle with Flaky Sea Salt - Sprinkle the sea salt over the top for contrast.
Step 8: Cool and Cut - Let cool completely, then cut into squares.