Preheat your oven to 350°F. Line two baking sheets with parchment paper and set aside.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a large bowl, cream the butter, brown sugar, and granulated sugar with an electric mixer on high speed until light and fluffy, about 2-3 minutes. (You can use a stand mixer with a paddle attachment if preferred.)

Add the egg yolks and vanilla extract, mixing on medium speed until pale and fluffy, about 1-2 minutes. Scrape the sides of the bowl as needed.

Add the dry ingredients to the wet mixture and mix on low speed just until combined.

Fold in the oats and cornflakes with a rubber spatula.

Scoop the dough with a large, 2-tablespoon capacity cookie scoop and place the dough balls 2 inches apart on the prepared baking sheets.

Bake for 10-12 minutes. (10 minutes for chewy cookies, 12 for crispier ones.)

Let the cookies cool on the baking sheet for five minutes, then transfer them to a wire rack to cool completely.

While the cookies are still warm, use a large circular cookie cutter to shape them perfectly. Sprinkle flaky sea salt on top while they cool.