Prepare a 9x13-inch pan by buttering it thoroughly and lining it with parchment paper, leaving overhangs for easy removal. Lightly butter the parchment paper as well.

In a large, heavy-bottomed saucepan, combine the butter, brown sugar, heavy whipping cream.

Over medium low heat, whisk constantly until butter melts and the mixture begins to bubble. Cook for 5-7 minutes until caramel coats the back of a spoon. Alternatively, attach a candy thermometer and cook the caramel mixture until it reaches 238°F (soft-ball stage), ensuring it doesn’t exceed this temperature to avoid hardening later.

Remove the caramel from the heat and stir in the sea salt until well incorporated.

Immediately add the mini marshmallows to the caramel mixture and stir until completely melted and smooth.

Quickly fold in the Rice Krispies cereal, ensuring all pieces are evenly coated with the marshmallow-caramel mixture.

Transfer the mixture into the prepared pan. Using a buttered spatula or your hands (lightly greased with butter), press the mixture evenly into the pan without compacting it too tightly. Allow to cool for 30 minutes.

Drizzle the caramel sauce over the top of the Rice Krispie treats and sprinkle with a pinch of flaky sea salt.

Allow the treats to continue to cool at room temperature for at least 30 more minutes before slicing into squares or rectangles.