Grease an 8x8-inch baking pan with non-stick spray and line with parchment or foil. Sprinkle 3 tablespoons of cinnamon sugar across the bottom.

In a large microwave-safe bowl, combine butter, sea salt, and mini marshmallows. Microwave in 30-second intervals until fully melted, about 1-2 minutes.

Stir in vanilla extract once the mixture is melted and smooth.

Pour in Cinnamon Sugar Rice Krispies and plain Rice Krispies, gently folding them into the mixture until evenly coated.

Fold in 2 cups of remaining mini marshmallows until evenly distributed.

Transfer the mixture into the prepared pan, using a buttered spatula to spread and press down evenly.

Sprinkle remaining cinnamon sugar on top, then let cool at room temperature for about 30 minutes.