Warm large sauce pan to medium heat and add butter. Once the butter has completely melted, allow it to brown-- butter will foam and sizzle as brown specks appear on the bottom of the pan. When the aroma of the pan is nutty and the bottom of the pan is a toasty-brown color, add all but 1 1/2 cups of the marshmallows and turn off heat. Stir together until smooth. If the marshmallows are still not completely melted once the butter and marshmallows have thoroughly combined, turn your pan to low heat and stir until marshmallows are melted. Remove from heat.
In an extra large mixing bowl combine cereal, 1 teaspoon of the flake salt, and marshmallow mixture into cereal bowl and mix together with a spatula. After mixture is cooled, add remaining marshmallows and mix together.
Spray a 9x13 inch baking dish with baking spray. Press cereal and marshmallow mixture into dish with spatula or a bench scraper until the top is a smooth, even layer. Top with remaining Jacobsen salt (1/2-1 teaspoons). Cool completely before cutting into 12 squares. Wrap leftovers in plastic wrap or an airtight container.