6 cups crispy rice cereal (such as Rice Krispies®)
Grease an 8x8x2-inch pan with cooking spray.
Melt butter in a large pot over medium heat. Add sweetened condensed milk. Stir with a rubber spatula until butter and milk are uniformly combined and small bubbles are forming in the center of the mixture, about 2 minutes. (When you first begin stirring they will look separated.) Add the bag of marshmallows and stir until melted and smooth. Add vanilla extract and salt and stir again. Remove from heat.
Stir rice cereal and remaining cup of marshmallows into the pot to combine. Transfer to the prepared pan and press down lightly with a sheet of plastic wrap; don’t pack them down too hard or they will lose their inherently soft and gooey texture. Sprinkle with flaky sea salt.
Let set up on the counter for at least 1 hour. Cut into 16 squares.