Line a 9 x 9-inch baking pan with parchment paper or foil and spray with nonstick cooking spray.

In a large pan, melt butter over low heat. Set aside 2 cups of marshmallows. Add the rest of the marshmallows to the pan and stir until completely melted. Remove the pan from the heat. Stir in the sea salt and vanilla and mix until incorporated.

Gradually add Rice Krispies cereal; stirring until well coated. Fold in the reserved 2 cups of marshmallows until thoroughly incorporated.

Using a buttered spatula or clean hands to gently press the mixture into the prepared pan. Don’t push too firmly or you’ll lose a bit of the ooey-gooey factor that we’re looking for. Cool completely at room temperature.