Preheat your oven to 350°F and prepare large baking sheets by lining them with parchment paper. In a separate medium bowl, combine the flour, baking powder, baking soda, and salt, whisking together until evenly blended. In a large bowl or the bowl of a stand mixer equipped with a paddle attachment, beat the butter, granulated sugar, and brown sugar on medium speed until the mixture becomes light and fluffy, approximately 3 minutes. Incorporate the egg and vanilla extract, mixing thoroughly and scraping the bowl's sides as needed. Gradually add the dry ingredients to the wet mixture, blending on low speed just until combined. Carefully fold in the cornflakes, oats, and chopped nuts, ensuring they are evenly distributed while preserving the texture of the cornflakes. Using a 1½ tablespoon cookie scoop, portion the dough onto the prepared baking sheets, spacing them about 2 inches apart. Optionally, press extra cornflakes onto the tops of the dough balls for added crunch. Bake each tray individually for 10 to 12 minutes, or until the cookie edges turn lightly golden. Allow cookies to cool on the sheet for several minutes before transferring them to a wire rack to cool completely. Store in an airtight container at room temperature for up to one week, or freeze for up to a month.