Line a 9-inch x 13-inch pan with parchment paper.
Spray with cooking spray. Set aside.
In a large pot over medium low heat, melt butter and cook, stirring constantly, until browned, about 5 minutes.
Add 16 ounces marshmallows.
Stir until completely melted.
Remove from heat.
Add vanilla, cereal, and 4 ounces marshmallows.
Stir until thoroughly coated.
Quickly press into prepared pan.
Allow to set before cutting.