Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

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

In a large mixing bowl, cream together the butter, brown sugar, and white sugar until fluffy (2-3 minutes).

Add the egg yolks and vanilla extract to the creamed mixture, mixing until pale and fluffy (1-2 minutes).

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined.

Fold in the oats and cornflakes gently until evenly distributed.

Use a cookie scoop to drop rounded portions of dough onto the lined baking sheets, spaced about 2 inches apart.

Bake for 10-12 minutes, until lightly golden brown around the edges.

Allow the cookies to cool on the baking sheet for about 5 minutes before transferring to a wire rack.