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.
4. If covered and stored at room temperature, these brownies should keep for 3 to 4 days.