Preheat the oven to 350F. Line a baking sheet with parchment paper or a silicone mat.
In the bowl of a stand mixer fitted with the paddle attachment or with a hand held electric mixer, beat the butter and sugars until fluffy.
Beat in the egg and egg yolk until combined.
Add the vanilla and the Nutella. Mix until combined.
Add the flour, cocoa powder, baking soda, and salt. Stir just until incorporated.
Fold in the chocolate chips.
Using a medium cookie scoop (1.5 tablespoons), drop dough 2-inches apart onto the prepared baking sheet.
Bake for 8-13 minutes, or until edges look set and the centers look soft.
Cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Notes
Butter: Cool room temperature means that the butter is about 68F. The butter should dent when pressed, but it shouldn't be warm enough that it's soft or smooshy.
Flour: To properly measure flour, stir the flour, lightly spoon into a measuring cup, and then level. This helps to avoid packing in too much flour.
Chocolate chips: Can sub milk or bittersweet chocolate chips.