Preheat your oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a medium bowl, whisk together buckwheat flour, Dutch-process cocoa powder, and salt until well combined. Set aside.
In a large mixing bowl, melt unsalted butter or coconut oil in the microwave or on the stovetop. Allow it to cool slightly.
Add granulated sugar, pure vanilla extract, and peppermint extract to the melted butter. Whisk until fully combined.
Add large eggs (at room temperature) to the mixture. Whisk until the eggs are fully incorporated and the mixture is smooth.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
Gently fold in semi-sweet chocolate chips or desired mix-ins until evenly distributed.
Pour the brownie batter into the prepared baking pan, smoothing the top with your spatula. Bake 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 for about 10 minutes before lifting them out using the parchment paper overhang. Transfer to a wire rack to cool completely before slicing into squares.