Pre-heat your oven to 350F (conventional). Add the oats to a large baking pan and toast in the oven for 7-8 minutes.

In the meantime, place the butter in a non-stick pan and melt it on medium-low heat. Keep stirring for a few minutes until the butter starts to look brown and develops a caramel-y nutty aroma.

Turn the heat all the way to the lowest setting, and add the brown and white sugars. Whisk on the lowest flame for just 1-2 minutes, then remove from the heat.

Remove from heat and cool the mixture for 5 minutes. It should be warm, but not hot anymore.

Add the egg, egg yolk, and vanilla, and whisk for 1-2 minutes until the mixture becomes very smooth and creamy like caramel.

Sift the flour, cinnamon, salt, baking powder and baking soda into a separate bowl. Add the toasted oats and mix everything together.

Add the dry ingredients to the wet in 2 batches and fold with a rubber spatula until just combined. Do not overmix!

Lastly, gently fold in the chopped chocolate (ensure the mixture isn't hot beforehand).

Use a 2 tablespoon cookie scoop to divide the dough into 10 balls (if the dough feels a bit too runny to scoop, refrigerate it for 10 min first).

Place the cookie balls onto a plate or baking sheet, and add a few chocolate chunks on top.

Refrigerate for at least 2 hours, but ideally overnight for best results.

Pre-heat oven to 370F (conventional / no fan) and prepare a large light-colored cookie sheet with parchment paper.

Transfer the balls to the sheet, keeping at least 3" between them.

Bake for 10-12 minutes depending on how soft and gooey you want the cookies to be.

Sprinkle the cookies with some flakey sea salt (optional) and let them cool down at room temperature for 10-15 minutes.

Serve and enjoy while still warm!