Line a baking sheet with parchment paper. Using a small ice cream scoop or a round measuring tablespoon, scoop out 16 rounded tablespoons of Nutella onto the prepared baking sheet. Freeze until solid for at least 6-8 hours. Line another baking sheet with parchment paper.
Mix flour, cornstarch, baking soda and salt together in a medium bowl and set aside.
Beat the butter on medium high speed until soft and slightly lightened in color, about 1 minute. Add both sugars and beat together until lightened in both color and texture, about 5 minutes.
Mix in the egg, egg yolk and vanilla until well blended.
On low speed, mix in the cocoa powder. Add the flour mixture and mix until just combined. Do not over mix.
Scoop out the dough onto the lined baking sheet, forming 16 equal dough balls. Using the back of a wooden spoon handle or your thumb, make holes into the dough.
Get the Nutella scoops out of the freezer and working quickly, peel the Nutella scoops from the baking sheet and place them in the middle of the dough holes. If you’re working in a warm kitchen, you might want to keep them in the freezer, taking them only one by one as you work, to avoid them melting.
Gather the dough up over the Nutella scoops to completely cover them. Loosely cover with plastic wrap and freeze for 6-8 hours.
Preheat oven to 180C and adjust oven rack to middle position. Bake until the cookies flatten with a slight dome, and the outer edges start to harden, yet centres are soft and puffy, 11 to 14 minutes. Start checking at 11 minutes. The centres might feel undone. DO NOT OVER-BAKE or you’ll get hard cookies. The cookies will continue to bake after they come out of the oven from the residual heat.
Sprinkle sea salt on top. Let the cookies cool on the baking sheet for at least 15 minutes before serving.