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.
¾ cup unsalted butter, ¾ cup brown sugar, ½ cup granulated sugar
Beat in the egg and egg yolk until combined.
1 large egg, 1 egg yolk
Add the vanilla and the Nutella. Mix until combined.
1 teaspoon vanilla extract, ½ cup Nutella
Add the flour, cocoa powder, baking soda, and salt. Stir just until incorporated.
¼ cup unsweetened cocoa powder, 2 ½ cups all-purpose flour, 1 teaspoon baking soda, ½ teaspoon salt
Fold in the chocolate chips.
1 cup semisweet 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.