Line a large baking sheet with wax paper. Place the cornflakes in a large bowl.
In a small saucepan over medium-high heat, bring the sugar and corn syrup just to a boil, stirring with a small spatula to dissolve the sugar, about 2 to 3 minutes. Remove from the heat and stir in the peanut butter until incorporated.
Pour the peanut butter mixture over the cornflakes and use the spatula to gently toss to coat.
Working quickly, drop spoonfuls of the mixture onto the paper, and let them set, about 10 minutes, before serving.
