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Heat large pot or dutch oven over medium-low heat. Cut butter into 1 tablespoon pieces, then add to pot. Cook butter, stirring occasionally, for about 5-10 minutes or until it has reached your desired browness. It should have brown pieces in it and it should smell toasty.
Add marshmallows to pot. Cook while stirring constantly until marshmallows are completely melted. Remove pot from heat.
Add Rice Krispies to pot and fold in until eventually coated with butter/marshmallow mixture. Evenly spread mixture into greased 9x11-inch baking dish and sprinkle with flaky salt. Let cool for at least one hour before cutting into pieces.