Lightly grease a 9x13-inch (23x33 cm) baking pan with butter or non-stick spray. Set aside.

In a large saucepan, melt the butter over low heat. Add the mini marshmallows and stir continuously until completely melted and smooth.

If using, add vanilla extract and mix well.

Remove the pan from heat. Immediately add the Rice Krispies cereal and gently fold until the cereal is fully coated with the caramel-marshmallow mixture.

Transfer the mixture to the prepared pan. Using a buttered spatula or wax paper, press the mixture evenly into the pan. Avoid pressing too hard to keep the treats light and crispy.

Let the treats cool at room temperature for about 1 hour or until set. Once set, pour the caramel sauce over the top and spread it out. Sprinkle with flake salt. Cut into squares.