In a large pot, melt the butter over medium-low heat and stir in the peanut butter until well blended.
Add 9 oz of mini marshmallows to the butter mixture, stirring until fully combined.
Incorporate the cornflakes, ensuring they are evenly coated.
Gently fold in the remaining mini marshmallows.
Line a pan with parchment paper.
Scoop out the mixture and shape it into cookies.
Allow the cookies to cool for 10-20 minutes.