Line two baking sheets with parchment paper or silicone baking mats.
In a large saucepan over medium heat, combine the granulated sugar, corn syrup, and peanut butter.
Cook, stirring frequently, until the mixture comes to a boil.
Continue boiling for exactly 2 minutes, stirring often.
Remove the saucepan from the heat and stir in the vanilla extract.
Add the cornflakes cereal and gently fold until evenly coated.
Using a cookie scoop, portion the mixture into 2- to 3-tablespoon mounds onto the prepared baking sheets.
Allow the cookies to rest at room temperature for 2 hours, or until fully set.
Serve or store in an airtight container.
Christina's Notes
Boil the mixture for exactly 2 minutes. Undercooking can prevent the cookies from setting properly, while overcooking can make them hard.
Work quickly once the cornflakes are added, as the mixture begins to firm up as it cools.
Fresh cornflakes provide the best texture and crunch.
Shelf-stable creamy peanut butter works best. Natural peanut butter may cause the cookies to set inconsistently.
Store in an airtight container at room temperature for up to 5 days.
Freeze in a freezer-safe container for up to 2 months.