Measure out your cornflakes into a large bowl, being careful not to crush them too much.
In a saucepan over medium heat, combine the corn syrup and sugar. Stir the mixture occasionally until the sugar is completely dissolved.
Once the sugar has dissolved, turn off the heat and stir in the peanut butter and vanilla extract. Mix until it is smooth and well combined.
Pour the peanut butter mixture over the cornflakes. Use a rubber spatula or wooden spoon to gently fold the cornflakes into the mixture, making sure every flake is coated.
Using a cookie scoop or two spoons, scoop out portions of the cornflake mixture onto a parchment-lined baking sheet. Flatten each mound slightly to resemble a cookie shape. Allow them to cool completely, they will harden as they cool.
While the cookies are cooling, make your chocolate drizzle. In a microwave-safe bowl, combine the semi-sweet chocolate chips and vegetable oil. Microwave in 15-second intervals, stirring after each, until the chocolate is melted and smooth.
Once the cookies have hardened, use a spoon to drizzle the melted chocolate over each one. Let the chocolate set before serving.