Preheat oven to 350 degrees F.
In a stand mixer or with a hand mixer, cream the granulated sugar, brown sugar and butter with a paddle attachment on medium speed until the mixture looks smooth. Do not over cream so that the butter gets too light and fluffy.
Reduce the speed to low, slowly add the egg and vanilla and mix until fully incorporated.
Scrape the bottom of the bowl with a spatula, add the chocolate chips and hazelnuts and mix for an additional 15 to 30 seconds until combined. Add the flour, salt, baking soda, and cornstarch, and continue to mix on low speed until just combined.
Using a 2 ounce cookie scoop, scoop the cookies onto the parchment-lined baking sheets 2 to 3-inches apart.
Make a divot in the center of the cookie to hold the chocolate hazelnut spread and spoon some of the spread into each cookie. Take an additional scoop of dough and press it into a flat disc. Cover the hazelnut spread half and sandwich the top pressing along the edges to encase the cookie.
Flatten slightly with the palm of your hand, add additional extra chocolate chips on top for garnish and a sprinkle of Maldon sea salt. Bake for 12-15 minutes until golden around the edges.
Remove from the oven and let cool for about 5 minutes then place on a rack to cool completely. Serve or place in an air-tight container with parchment for up to a week.