In a large pot over medium-low heat, melt the butter and peanut butter together. Stir in 10 oz of mini marshmallows, constantly stirring until completely melted and smooth. Remove from heat.
Gently fold the cornflakes into the melted mixture, ensuring they are fully coated. Add the remaining 1 oz of mini marshmallows and the optional ⅓ cup peanut butter chips. Stir gently to combine, taking care not to overmix.
Spray a large cookie scooper with nonstick spray and scoop out 10 equally sized peanut butter cornflake balls onto a parchment-lined baking sheet. Grease your fingers and gently press each cookie to about 1 inch in thickness without packing them too tightly.
Melt 2 tbsp peanut butter chips with ½ tsp oil in the microwave, heating in 30-second increments and stirring between each interval until smooth. Transfer the melted peanut butter mixture to a ziplock bag, cut a small opening at the tip, and drizzle over each cookie.
Allow the cookies to set for 20-30 minutes before serving.