Prep the pan. Heat the oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease lightly.
Combine dry ingredients. In a large bowl, whisk the flour, cocoa, baking powder, baking soda, and salt until evenly mixed.
Mix the sugars and wet ingredients. In a separate bowl, whisk the granulated sugar, brown sugar, eggs, oil, yogurt, orange juice, orange zest, and vanilla until smooth.
Bring it together. Pour the wet mixture into the dry ingredients.Stir gently with a spatula until just combined. A few small lumps are fine.
Add the chocolate. Fold in the chocolate chips or chunks, reserving a small handful for the tops.
Fill the cups. Divide the batter evenly among the muffin cups, filling each about 3/4 full. Sprinkle on the reserved chocolate and coarse sugar if using.
Bake hot, then finish. Bake at 400°F (200°C) for 5 minutes to boost the rise.Without opening the oven, reduce the temperature to 350°F (175°C) and bake 12–14 more minutes, until a toothpick comes out with a few moist crumbs.
Cool. Let the muffins sit in the pan for 5 minutes, then move to a wire rack to cool. Enjoy warm or at room temperature.