In a large bowl, whisk together flour, soda, cornstarch, and salt.

In another medium bowl, use a whisk or electric beater to beat together butter and sugars until no lumps remain. Add egg, yolk, and vanilla extract and continue to beat until smooth. Slowly stir in flour mixture until batter comes together. Stir in chopped chocolate.

Cover and chill dough for at least 2 hours or overnight (or over several nights. It just gets better and better).

When ready to bake, preheat oven to 325 degrees. Line 2 large cookie sheets with parchment paper.

Scoop heaping teaspoons of half the dough onto the lined baking sheets. Cookies should be 1 1/2 inches apart. Use your thumb to press a well into the center of the dough. Spoon the chocolate hazelnut spread into the centers. Scoop the remaining half of the dough on top. Do not flatten.

Bake cookies for 11-13 minutes, rotating the pans halfway through. The cookies will look soft and underdone. That’s okay. Allow them to set up on the baking sheet for about 5 minutes then remove to a wire rack to cool completely.

Cookies keep in an airtight container for up to a week.