Preheat oven to 350 degrees F and line a 9x9 baking pan with parchment paper or spray with non-stick baking spray, set aside.
Add butter and 70% dark chocolate to a microwave-safe large mixing bowl. Heat in the microwave for 1 minute. Remove from the microwave and stir. Add the bowl to the microwave and heat again for 15-second intervals, stirring in between, until the chocolate is completely melted and smooth.
Add the granulated sugar and stir to combine.
One at a time, add your eggs and stir until combined and the batter is shiny.
Add the instant espresso powder and vanilla bean paste and stir to combine.
In a small bowl, whisk together buckwheat flour, white whole wheat flour, and kosher salt.
Add the flour mixture to the wet ingredients and stir until the flour is incorporated.
Add in 3/4 cup chocolate chips and 3/4 cup chopped hazelnuts and stir to combine. The batter will be thick.
Add batter to the prepared dish and top with remaining chocolate chips and chopped hazelnuts.
Bake for 40 minutes or until the brownies are fully cooked. The sides should be fully set with a set but still slightly soft middle. Cook to 165 degrees F internally for fudgy brownies and 210 degrees F internally for more cake like brownies. I think these brownies work best when cooked to the fudgy stage.