Preheat the oven to 350°F. Line large baking sheets with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugars on medium speed until light and fluffy, about 3 minutes. Add the egg and vanilla and beat well, stopping as needed to scrape down the sides during mixing.
Add the flour mixture and beat on low speed just until combined. Gently fold in the cornflakes, oats, and nuts until well incorporated into the dough. Using a 1½ tablespoons cookie scoop, drop the dough onto the prepared baking sheets about 2 inches apart. Press more cornflakes to the outside of the dough balls, if desired.
Bake one sheet at a time for 10 to 12 minutes or until the edges start to brown. Let the cookies cool on the baking sheet for a few minutes, then transfer to a wire rack to cool completely. Store cookies in an airtight container at room temperature for up to 1 week or frozen for up to 1 month.
Notes
Be gentle when folding in the cornflakes, oats, and nuts, so the cornflakes don’t get totally crushed up.
A handheld mixer can be used if a stand mixer is not available.
I highly recommend using a cookie scoop. Using a cookie scoop ensures that all the cookies are the same size so that they bake evenly. If you bake cookies regularly, it’s worth having one on hand.
Room temperature butter and egg will mix more evenly. If your egg is cold, place it into a bowl of warm water for a couple of minutes. Doing so will quickly bring the egg to room temperature.
Adding too much flour will lead to the cookies being dense. A kitchen scale is the best and easiest way to measure flour. If you don’t have one, then fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it.
Don’t be too rough when you mix in the cornflakes and oats, as you don’t want to crush the cornflakes.
Make sure to line your sheet pan with parchment paper or a silicone mat. A lined cookie sheet helps the cornflake cookies bake more evenly. Also, if you add the cookie dough directly onto a cookie sheet, they’ll stick, and the bottoms will be darker and more crispy.