In a large saucepan over medium heat, add the 8 tablespoons of unsalted butter. Stir occasionally until the butter is completely melted and bubbly.
Once the butter is melted, add the 16-ounce bag of mini marshmallows to the saucepan. Stir continuously until the marshmallows are fully melted and smooth.
Remove the saucepan from heat and stir in the 1 teaspoon of vanilla extract and 1/2 teaspoon of sea salt.
In a large mixing bowl, combine the 4 cups of crispy rice cereal and 4 cups of Cinnamon Toast Crunch cereal. Pour the melted marshmallow mixture over the cereals and quickly mix until everything is evenly coated.
Gently fold in the 2 cups of additional mini marshmallows for extra gooeyness.
Line your 9x13-inch baking dish with parchment paper or grease it lightly. Transfer the mixture into the dish and use your spatula to press it down evenly.
Lightly sprinkle a pinch of sea salt on top of the pressed treats.
Let the treats sit at room temperature for at least 30 minutes to set. Once set, cut them into squares or rectangles for serving.
