Preheat the oven to 350°F. Line an 8"x8" pan with parchment paper, leaving an overhang on two sides for easy removal.

In a medium bowl, whisk together 1 cup buckwheat flour, 3/4 cup Dutch-process cocoa powder (sift if lumpy), and 1/4 teaspoon salt. Set the dry mixture aside.

Measure 3/4 cup + 1 tablespoon unsalted butter or coconut oil, melt it, and let it cool slightly so it is warm but not hot.

In a large bowl, combine the cooled melted butter or oil with 1 1/2 cups granulated sugar, 1 teaspoon pure vanilla extract, and 2 teaspoons peppermint extract. Stir until the sugar is evenly moistened.

Add the 3 large eggs, one at a time, stirring after each egg just until incorporated. Do not overmix.

Add the dry mixture to the wet ingredients and stir gently until almost no streaks of flour remain. Stop as soon as the batter is uniform—do not overmix.

Reserve 1/4 cup of the 1 1/4 cups semi-sweet chocolate chips for topping. Fold the remaining 1 cup chocolate chips (or other add-ins) into the batter.

Pour the batter into the prepared pan and smooth the top with a spatula. Evenly sprinkle the reserved 1/4 cup chocolate chips over the batter.

Bake for 16–20 minutes, or until a thin crust has formed and the center appears set. A toothpick inserted into the center should come out with a few moist crumbs but not wet batter.

Remove the pan from the oven and place it on a wire rack. Let the brownies cool completely in the pan; they will continue to firm up as they cool.

When cool, use the parchment overhang to lift the brownies from the pan and cut into squares. Cover and store at room temperature for up to 4 days.