Line two baking sheets with parchment paper or silicone baking mats.
In a large saucepan, combine the sugar, corn syrup and peanut butter over medium heat.
Stir often until the peanut butter mixture comes to a boil. Continue boiling for 2 minutes, stirring frequently, then remove from heat and stir in the vanilla and cornflakes.
Use a cookie scoop to portion out the cornflake mixture in 2-3 tablespoons sized cookies onto the prepared baking mats.
Let set for 1-2 hours before storing or serving.
Christina's Notes
Timing: Boil for exactly 2 minutes. If the mixture doesn't cook for long enough, they won't set, and if it boils for too long, they'll be hard.
Work Quickly: After you stir in the cornflakes, portion quickly before the mixture stiffens.
Fresh Cereal: The crisper the flakes are, the crisper the cookies will be.