►Heat oven to 325°F and line a 9-inch-by-9-inch baking dish with parchment paper.

►Combine flour, sugar, cocoa powder, and salt in the bowl of a food processor and pulse until combined. Add butter, a few chunks at a time, pulsing between additions, until dough pulls away from sides of bowl.

►Transfer dough to prepared baking dish and press with your fingers so dough evenly covers bottom of dish. Refrigerate for at least 20 minutes (and up to 3 days if covered with plastic wrap).

►Transfer dish to oven and bake for 35 to 40 minutes, until firm to the touch. Cool on a rack for 20 minutes.

►While shortbread is cooling, warm crème fraîche in a small saucepan over medium heat. Whisk until smooth and bring to a simmer. Put chocolate in a medium bowl. When crème fraîche is fully melted and slightly steaming, pour over chocolate. Let sit for 2 minutes, then whisk into a smooth ganache.

►When shortbread is cool, spread ganache over the top. Let set for about 30 minutes, then sprinkle with chopped hazelnuts. Let set for another 30 to 60 minutes, then refrigerate for about 20 minutes before cutting into 16 bars.