Preheat oven to 350°F. Line two sheet pans with parchment.
Whisk flour, cocoa, baking soda, and salt in a medium bowl.
Beat butter, chocolate-hazelnut spread, granulated sugar, and brown sugar until fluffy, about 2 minutes.
Mix in vanilla and the beaten egg until smooth.
Stir in dry ingredients on low just until no dry streaks remain.
Fold in chocolate chips and hazelnuts if using.
Scoop heaping tablespoon portions onto pans, spacing about 2 inches apart.
Bake 9–11 minutes, until edges set and centers look soft.
Cool 5 minutes on the pan, then transfer to a rack to finish cooling.