Prep the pan and oven: Heat the oven to 375°F (190°C).
Line a 12-cup muffin tin with paper liners or lightly grease the cups.
Combine dry ingredients: In a large bowl, whisk together buckwheat flour, cocoa powder, baking powder, baking soda, salt, and espresso powder (if using) until evenly mixed.
Mix the wet ingredients: In a separate bowl or large measuring cup, whisk the brown sugar, granulated sugar, yogurt, milk, eggs, oil, and vanilla until smooth and well combined.
Add the melted chocolate: Melt the chopped chocolate gently in the microwave in short bursts or over a double boiler. Let it cool for 1–2 minutes, then whisk it into the wet mixture.
Bring it together: Pour the wet mixture into the dry ingredients. Use a spatula to fold just until the flour disappears.
If using chocolate chips, fold them in now. The batter will be thick and a bit lumpy—perfect.
Fill the cups: Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full. Sprinkle the tops with coarse sugar or a few extra chips if you like.
Bake: Bake for 16–20 minutes, or until the tops are set and a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).
Cool: Let the muffins rest in the pan for 5 minutes, then transfer them to a wire rack to cool.
Enjoy warm for melty chips or at room temperature for a tidier crumb.