In a large pot over medium-low heat on the stovetop, melt the butter. Once melted add the marshmallows, stirring constantly until melted.
Add the peanut butter and vanilla extract, stirring until melted. Remove from heat.
Stir in the cornflakes until well coated.
Use two spoons to create mounds of the cornflake mixture and place them onto a parchment paper lined baking sheet, approximately 1/4 cup to each cookie.
Allow the cookies to solidify on the counter or in the refrigerator for at least an hour before serving.
Notes
Store the fully set cookies in an air-tight container kept at room temperature for 4-5 days.