- Preheat the oven to 350°F. Butter a 8- to 10-inch square pan and set aside.
- Melt the butter and chocolate together in a medium saucepan over low heat.
- In a small bowl, whisk together the flour, cocoa powder, baking powder and salt.
- In the bowl of a standing mixer (or using a large bowl and a hand mixer), beat the eggs with the sugar until pale in color and thicker in texture, about 4 minutes.
- Beat in the vanilla extract and coffee.
- Slowly add the flour mixture and mix just until combined. Then fold in the melted chocolate and hazelnuts.
- Scrape the brownie batter out into the prepared baking dish.
- Transfer to the oven and bake for about 45 minutes, or until the top becomes a bit cracked and — with any luck — slightly shiny. The center of the pan of brownies should still be a touch jiggly — they firm up a good bit as they cool. Let cool completely before slicing.