Preheat the oven to 350 degrees and line two cookie sheets with parchment paper. In a medium bowl combine flour, cocoa powder, baking soda, cornstarch, and salt until well mixed. Set aside.

In the bowl of your stand mixer (or a large bowl with a hand mixer) beat butter and both sugars until light and fluffy, 3-5 minutes. Beat in egg and egg yolk until fully incorporated, followed by the vanilla.

On the lowest setting of your mixer, stir in the flour mixture until dough forms.

Scoop out rounded tablespoons of dough and, using your thumb, push into the center to make an indentation. Place cookies (thumbprint side up) at least 2 inches apart on prepared cookie sheets.

Bake at 350 degrees for 12-14 minutes or until edges just begin to brown. Remove from oven and let cook on cookie sheets for a few minutes before carefully removing to a wire rack to cool completely.

Once cool, scoop nutella into a microwave-safe plastic bag and microwave for 15 second intervals until nutella is soft and pipe-able. Snip off a corner of the plastic bag and fill centers of cookies with nutella, approx 1 1/2 tsp per cookie. Feel free to add more or less as you see fit, though.

Top with sprinkles and serve!