In a medium saucepan over medium heat, melt 4 tablespoons of butter until completely liquid. Stir in 1 cup of brown sugar and 1/4 cup of corn syrup until well combined and bubbling slightly, about 2-3 minutes.

Remove the saucepan from the heat and carefully add 1/2 cup of heavy cream, mixing continuously until smooth.

Add 4 cups of mini marshmallows to the caramel mixture, stirring constantly over low heat until fully melted, about 3-4 minutes.

Fold in 6 cups of Rice Krispies into the warm mixture until evenly coated. Allow to cool for 5 minutes.

Grease a 9x9 inch baking pan and transfer the Rice Krispie mixture, spreading it out evenly. Press down firmly with a spatula.

Sprinkle a generous pinch of finishing salt over the top and let the treats cool completely at room temperature for at least 1 hour.

Once cooled, slice them into squares or rectangles and serve.