Line a 9-by-13-inch pan with heavy-duty aluminum foil and lightly coat with softened butter. Reserve 2 cups of mini marshmallows.

In a large, light-colored pot or Dutch oven, melt the butter over medium heat, swirling occasionally to ensure even cooking. Watch for the butter to change from yellow to golden to a deep, toasty brown with a nutty aroma. Remove from heat once browned; if the sediment becomes nearly black, strain the butter through a fine mesh sieve before returning it to the pot.

Stir in the remaining marshmallows, salt, and vanilla off the heat. Return the pot to medium heat and stir continuously until all marshmallows are fully melted.

Take the pot off the burner and incorporate the cereal, stirring thoroughly with a wooden spoon or spatula until well combined.

Add the reserved marshmallows and mix gently to preserve pockets of gooey texture.

Transfer the mixture into the prepared pan and use the butter wrapper or damp hands to press it evenly without compacting. Let sit at room temperature for at least one hour.

Lift the set treats from the pan using the foil, and cut into squares using a serrated knife. Store in an airtight container at room temperature for up to five days.