Line a baking sheet with parchment paper. Using a round measuring tablespoon measuring, scoop out 16 rounded tablespoons of Nutella onto the prepared baking sheet. Freeze until solid, at least one hour or overnight.
Line another baking sheet with parchment paper.
Whisk together flour, cornstarch, baking soda and salt in a medium bowl and set aside
In a large bowl and using a hand mixer (you can also use a stand mixer), beat the butter on medium high speed until soft and slightly lightened in color, about 1-2 minutes, scraping down the sides as needed.
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 just until combined, leaving a few streaks of flour behind. Do not over mix.
Using a 1/4 cup measure ice cream scoop, scoop out the dough onto the lined baking sheet, forming 16 equal dough balls.
Using your thumb or the back of a spoon, make deep 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 only one by one as you work, to avoid them melting.
Gather the dough up over the Nutella scoops to completely cover them. Roll the dough into balls and place on the baking sheet.
Loosely cover with plastic wrap and freeze for at least 2 hours, then either bake immediately (from frozen) or transfer to a large zipper lock bag and freeze until needed, overnight or up to 1 month.
Preheat oven to 350F/180C and adjust oven rack to middle position.
Line 2 baking sheets with parchment paper. Place 8 cookie balls per sheet, spacing them at least 2 inches apart.
Bake until the cookies flatten with a slight dome, no longer have a hump in the middle, and the outer edges start to harden yet centers are soft and puffy, 11 to 14 minutes. Start checking at 11 minutes. The centers will feel undone, but they shouldn't be shiny or sticky. The cookies will continue to bake after they come out of the oven from the residual heat.
Let the cookies cool on the baking sheet for at least 15 minutes before serving. Or cool completely then store in an airtight container.