Prepare an 8x8-inch baking pan by spraying it with non-stick spray and lining the bottom with parchment paper. Sprinkle 3 tablespoons of cinnamon sugar over the parchment.

In a large, microwave-safe bowl, combine ½ cup of butter, ½ teaspoon of sea salt, and 8 cups of mini marshmallows. Microwave in 30-second intervals, stirring, until melted and smooth, about 1-2 minutes.

Stir in 1 teaspoon of pure vanilla extract until fully incorporated into the melted marshmallow mixture.

Fold in 3 cups of cinnamon sugar Rice Krispies cereal and 3 cups of regular Rice Krispies cereal with a spatula until well combined.

While warm, fold in the remaining 2 cups of mini marshmallows until evenly distributed.

Transfer the mixture to the prepared pan and press down firmly with buttered hands or parchment paper to shape.

Sprinkle the remaining cinnamon sugar over the top before cooling.

Allow to cool for about 30 minutes until firm, then slice into squares.