Line a 13x9-inch baking pan with parchment paper or foil.

In a large pot, melt butter over medium heat. Meanwhile, reserve 2 cups of the mini marshmallows and set them aside.

Once the butter is melted, add the remaining marshmallows, vanilla, and salt to the pot. Stir with a silicone spatula until fully melted, about 5 minutes. Remove from the heat.

Add the cereal and stir until fully combined (mixture will be very sticky). Stir in the reserved 2 cups of marshmallows until evenly dispersed but not fully melted. (This will give little pockets of marshmallow in the treats!)

Pour the mixture into the baking pan. Using the silicone spatula or the butter stick wrapper, press the mixture into an even layer in the pan. Let cool completely, about 1 hour.

Use the parchment to remove the treats from the pan. Lightly grease a sharp knife and cut into squares.