Prep your pan. Line a 12-cup muffin tin with paper liners.
Preheat the oven to 425°F (220°C). This burst of heat helps create tall, domed tops.
Mix dry ingredients. In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt until evenly combined.
Mix wet ingredients. In a separate bowl or large measuring cup, whisk milk, yogurt, oil, melted butter, eggs, and vanilla until smooth.
Combine gently. Pour the wet ingredients into the dry. Stir with a spatula until just combined.
A few streaks of flour are okay. Do not overmix or the muffins can turn dense.
Fold in chocolate chips. Reserve a small handful for topping and fold the rest into the batter with two or three light strokes.
Portion the batter. Divide evenly among the muffin cups, filling them about 3/4 full. Sprinkle the tops with the reserved chocolate chips and coarse sugar if using.
Bake high, then lower. Bake at 425°F (220°C) for 5 minutes, then reduce the oven temperature to 350°F (175°C) without opening the door. Continue baking for 12–15 minutes, until the tops are golden and a toothpick comes out with a few moist crumbs.
Cool briefly. Let muffins cool in the pan for 5 minutes, then transfer to a wire rack.
They’re great warm, when the chocolate is still soft.