Preheat oven to 350 degrees F. Line baking sheets with parchment paper.
In a medium bowl, whisk together flour, cornstarch, baking soda, nutmeg and salt. Set aside.
In bowl of electric mixer, cream together butter and sugars on medium-high speed until light and fluffly, about 3 minutes. Add eggs, 1 at a time and vanilla extract; mixing well after each addition. Scrape down bowl.
With mixer on low speed, slowly add dry ingredients. Mix until combined.
Add Nutella by spoonful until just incorporated so it’s streaked through dough and not fully blended in. With rubber spatula, stir in chocolate chips and hazelnuts, trying to maintain streaks of Nutella. Dough will be stiff.
Using a 2 tablespoon (#40) cookie scoop, drop dough onto prepared baking sheets, about 2 inches apart. Press down on top of cookie dough to flatten slightly.
Bake 12 – 14 minutes until cookies are lightly browned on edges. Remove from oven and immediately sprinkle tops with flaky sea salt and stud with additional chocolate chips /nuts while cookies are warm if desired.
Allow cookies to cool for 10 minutes on baking sheet before transferring to wire rack to cool completely.
Store in airtight container at room temperature for up to 5 days.