Line an 8x8 baking pan with parchment paper. Set aside.
Take a medium saucepan and place over medium-low heat on the stove. Add your butter and stir until melted. You then want to brown the butter, so lower the heat to low and stir occasionally. It should foam and sputter, and then you'll see small white pieces which are the milk solids. Let these turn a light brown and it should smell toasted and nutty.
3 tablespoons unsalted butter
Add in the marshmallow fluff, vanilla extract and salt. Use a rubber spatula to combine it with the butter over low heat. Keep stirring until it is completely combined.
7 oz marshmallow fluff, 1 teaspoon vanilla extract, ½ teaspoon kosher salt
Take the pan off the heat and pour in the rice krispie cereal. Use the rubber spatula to stir, making sure all of the cereal is covered in the melted marshmallow fluff.
5 cups rice krispie cereal
Pour this mixture into your prepared pan. Use the rubber spatula to even out the top. Let it rest for 2 hours, then you can lift it out of the pan and slice into squares.