In a bowl or stand mixer, cream the butter and sugars together until smooth. Add egg and vanilla. Mix until well combined.
Add flour, cocoa powder, baking soda, and salt. Mix until just combined. Careful NOT to over mix or you may end up with flat cookies. Fold in chocolate chips and chopped hazelnuts.
Using a medium cookie scoop, form cookie doughs into balls (~1 1/2 tablespoons size). Place the cookie dough balls on a baking sheet and freeze the dough balls for 20 minutes.
Preheat oven to 350ºF. Line a baking sheet with parchment paper and place dough balls on the baking sheet. Bake for about 10-11 minutes. Remove from oven, sprinkle with flaky sea salt. Enjoy!