Preheat oven to 350 degrees F. Line a baking sheet with parchment paper. Set aside.
In a bowl, cream the butter and sugar together until smooth. Add the egg, heavy cream, 3 ounces melted dark chocolate (save the rest for drizzling) and vanilla until combined. Add flour, cocoa powder, and salt. Mix until just combined.
Form 1” balls using a cookie scoop. I use this cookie scoop to make the dough balls so each cookie is the same size. Once I scoop, I roll them by hand to make a smooth ball. This helps keep them from cracking while in the oven. Place them about 1” apart on the baking sheet.
Bake for 10 minutes. Remove the cookies from the oven and gently press centers again with the back of a teaspoon to maintain the thumbprint. Let cool.
Once cookies are cooled, spoon salted caramel into the center of every cookie. Drizzle the cookies with the remaining melted dark chocolate then top with chopped hazelnuts. Let cookies set for about 15 minutes before serving. Enjoy!