Prep your pan and oven. Heat the oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease lightly.
Whisk the dry ingredients. In a large bowl, whisk flour, cocoa, sugar, baking powder, baking soda, and salt until no cocoa lumps remain.
Mix the wet ingredients. In a separate bowl, whisk eggs, buttermilk, oil, melted butter, and vanilla until smooth.
Combine gently. Pour the wet mixture into the dry. Stir with a spatula until just combined.
The batter should be thick and slightly lumpy. Do not overmix.
Fold in the good stuff. Add chocolate chunks and cherries. Fold gently to distribute without mashing the fruit.
Fill the muffin cups. Divide the batter evenly among the 12 cups. For high domes, fill to the top.
Sprinkle with coarse sugar if using.
Bake hot, then reduce. Bake at 400°F (200°C) for 5 minutes to help the muffins rise, then reduce the oven to 350°F (175°C) and bake 12–15 more minutes. They’re done when a toothpick comes out with a few moist crumbs (melted chocolate is okay).
Cool briefly. Let muffins rest in the tin for 5 minutes, then transfer to a rack. Serve warm or at room temperature.