Grease an 8x8-inch baking dish with butter or line with parchment paper for easy removal.
In a pot over medium-low heat, melt the butter. Stir frequently and cook until the butter turns golden brown and gives off a nutty aroma (about 4–6 minutes). Keep a close eye so it doesn’t burn.
4 TBS butter
Remove from the heat. Add the marshmallows and stir until completely melted and smooth.
5 oz Mini marshmallows
Stir in the vanilla extract and salt.
½ tsp vanilla, ¼ tsp salt
Quickly add the Cheerios and stir until evenly coated in the browned butter, marshmallow mixture.
3 cups Cheerios cereal
Transfer the mixture to your prepared pan. Lightly butter a spatula or your hands and gently press the mixture down evenly.
Let the bars cool for about 20 minutes before cutting into 9 equal squares.