Preheat the oven to 350°F and line a baking tray with parchment paper.
In a bowl whisk together brown sugar, white sugar, and melted butter.
1/4 cup brown sugar, 2 tbsp white sugar, 1/4 cup butter
Whisk in vanilla and egg yolk.
1 egg yolk, 2 tsps vanilla extract
Fold in flour and baking soda until a soft dough forms.
Toss in chocolate chips.
Divide the dough in half. Roll one half into a ball and press it flat into the center of a baking pan lined with parchment paper. It should resemble a hamburger patty.
Make a slight dent in the middle and fill with Nutella.
With the remaining dough, flatten and cover on top of the first half.
Gently seal the cookie, making sure there are no visible cracks of Nutella showing. See photo for reference.
Bake for 15-17 minutes or until the sides look golden brown. The center should look soft.
Allow to sit on the tray for about 15 minutes. The warmth from the tray will allow it to continue baking. Now would be the time to add extra chocolate chips if desired.
Sprinkle with flaky sea salt & enjoy!