Preheat your oven to 180°C and lightly grease a large cast-iron skillet (around 10 to 12 inches in diameter).
In a medium bowl, combine the flour, baking powder, baking soda, and salt. Stir well to mix.
In a large bowl, cream together the butter, brown sugar, and granulated sugar until light and fluffy. Beat in the egg, egg yolks, and vanilla extract until well combined. The mixture should look smooth.
Gradually add the dry ingredients to the butter and sugar mixture, mixing just until combined. Be careful not to over-mix. Stir in the chocolate chips.
Gently press the cookie dough into the prepared skillet, smoothing the top with a spatula.
Bake for 20 to 25 minutes or until the cookie is golden brown and the edges are set. The middle might seem a bit underdone, but it will continue to cook in the hot skillet after you remove it from the oven.
Remove the skillet from the oven and sprinkle the cookie lightly with flaked salt.
Allow the cookie to cool slightly, then cut into wedges or simply scoop directly from the skillet and serve warm. It's especially delicious with a scoop of vanilla ice cream on top!