Butter a 9x13-inch baking dish. Melt butter in a large saucepan over medium heat. Continue cooking to brown the butter. The butter will melt, then foam, then turn a golden color and finally start to brown and smell nutty. Stir frequently during cooking and remove the butter from the heat once it has browned. Meanwhile, place half the marshmallows on a parchment-lined baking sheet and broil on low 2-3 minutes or until toasted and golden brown. Return browned butter to low heat and add toasted and untoasted marshmallows, vanilla extract and salt. Cook, stirring often, until marshmallows melt completely and mixture turns light brown. Remove from heat and add cereal; stir using a silicone spoon or spatula, to coat. Pour mixture into prepared pan and press down lightly. Let bars cool completely before slicing and serving.