In a large bowl whisk together the dry ingredients - flour, baking soda, cornstarch and salt. Set aside.
In another large bowl using a stand mixer fitted with a paddle attachment or hand-held electric mixer beat softened unsalted butter until smooth and creamy, about 1 minute. Add granulated sugar and brown sugar and beat on medium-high speed until fluffy and pale yellow. Whisk in banana, egg, egg yolk and vanilla extract until combined. Beat in ¼ cup Nutella until combined.
Slowly mix the dry ingredients into the wet ingredients until combined. Add chocolate chips and mix until evenly distributed. Add the remaining ¼ cup Nutella and gently stir it in, until big streaks appear and not fully mixed. The dough will be thick and slightly sticky. Cover it tightly in aluminum foil or plastic wrap and chill for at least 2 hours and up to 2 days. Chilling is required.
Remove cookie dough from the refrigerator and let it sit at room temperature for 10-15 minutes. Preheat oven to 350°F. Line two large baking sheets with parchment paper. Set aside.
Scoop 1.5 tbsp cookie dough and roll into balls. Arrange 2-3 inches apart on prepared baking sheets.
Bake for 13-15 minutes or until slightly golden brown around the edges. Rotate the pan halfway through baking. If the cookies are too puffy, tap the baking sheet twice to help them spread. The cookies will look soft in the centers when you remove them from the oven.
Cool for 5 minutes on the baking sheet. Sprinkle with sea salt if desired, transfer to a cooling rack and let them cool completely.