Preheat oven to 350°F (177°C). Line two large baking sheets with parchment paper or silicone baking mats. Set aside.

In a large bowl using a handheld mixer or stand mixer fitted with a paddle attachment, beat the butter for 1 minute on medium speed until completely smooth and creamy. Add the brown sugar and granulated sugar and beat on medium high speed until fluffy and light in color. Beat in egg, egg yolk, and vanilla on high speed until combined. Scrape down the sides and bottom of the bowl as needed. Beat in 1/4 cup (74g) of Nutella until combined.

Whisk the flour, cornstarch, baking soda and salt together in a large bowl. On low speed, slowly mix into the wet ingredients until combined. Add 1 cup (180g) of chocolate chips and mix on low for about 5-10 seconds until evenly disbursed. Turn the mixer off and add the remaining Nutella. Beat for 5-10 seconds so the Nutella streaks through the cookie dough and is not fully mixed. Dough will be a little crumbly, that is OK. Dough will be extremely crumbly if the remaining Nutella was mixed in too much.

Scoop and roll balls of dough, about 1 Tablespoon of dough each, into balls. The warmth of your hands will help them come together. Arrange 3 inches apart on prepared baking sheets.

Bake the cookies for 10 minutes or until slightly golden brown around the edges. My oven has hot spots and yours may too—be sure to rotate the pan once during bake time. The baked cookies will look extremely soft in the centers when you remove them from the oven.

Cool for 5 minutes on the baking sheet. During this time, if the cookies are too puffy, try gently pressing down on them with the back of a spoon. They will slightly deflate as you let them cool. As the cookies cool, grab the remaining 1/4 cup of chocolate chips. Gently press a few chocolate chips into the tops of the slightly warm cookies (this is for looks and optional, of course!) and finish each with a sprinkle with sea salt. Transfer cookies to a cooling rack to cool completely.

Cookies stay fresh covered at room temperature for up to 1 week.