Step 1: Preheat your oven to 350°F (175°C) to ensure even baking.
Step 2: Line your 8x8 inch baking pan with parchment paper, leaving some overhang for easy removal.
Step 3: Melt the butter and dark chocolate together in a double boiler or microwave-safe bowl until smooth. Allow to cool slightly.
Step 4: In a mixing bowl, whisk together sugar, eggs, espresso powder, and vanilla bean paste. Stir in the melted chocolate mixture.
Step 5: Sift together buckwheat flour, white whole wheat flour, and salt in another bowl. Gradually mix into the wet ingredients until just combined.
Step 6: Fold in chocolate chips and chopped hazelnuts for added texture and flavor.
Step 7: Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick comes out with a few moist crumbs.
Step 8: Cool in the pan for 10 minutes, then lift out using the parchment paper and transfer to a wire rack to cool completely before slicing.