Start by preheating your oven to 350°F (175°C). In a medium saucepan, melt the 12 tablespoons of unsalted butter along with the 6 ounces of chopped dark chocolate over low heat. Stir frequently until the mixture is smooth and fully combined. Remove from heat and let it cool slightly.
In a large mixing bowl, whisk together the granulated sugar and the eggs until well combined. Add the 2 teaspoons of instant espresso powder and 2 teaspoons of vanilla bean paste to the mixture, stirring until smooth.
Once the chocolate and butter mixture has cooled slightly, pour it into the egg mixture. Stir until fully incorporated, ensuring no lumps remain.
In a separate bowl, whisk together the buckwheat flour, white whole wheat flour, and kosher salt. Gradually fold this dry mixture into the wet ingredients using a spatula until just combined.
Gently fold in the 1 cup of semisweet chocolate chips and 1 cup of chopped hazelnuts. Make sure they are evenly distributed throughout the batter.
Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal. Pour the brownie batter into the prepared pan and spread it out evenly. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan on a wire rack for at least 10 minutes. Once cooled, lift them out using the parchment paper and cut into squares. Enjoy your delicious Hazelnut Buckwheat Brownies!
