Grease a 9x13 pan with cooking spray.

In a large pot, melt butter over medium-low heat. When butter is melted, add in 1 1/2 bags of mini marshmallows, reserving 1/2 bag for later. Add salt.

When marshmallows are just melted (Don't cook too long!), remove from heat and stir in cereal and remaining mini marshmallows. Stir gently until evenly mixed.

Press evenly into prepared pan. Cool completely before cutting into squares and eating.