Preheat and prep: Heat the oven to 425°F (220°C). Line a 12-cup muffin tin with paper liners or grease well.

Whisk dry ingredients: In a large bowl, whisk flour, baking powder, baking soda, salt, and sugar until evenly combined.

Mix wet ingredients: In a separate bowl or large measuring cup, whisk eggs, buttermilk, oil, and vanilla until smooth.

Combine gently: Pour wet ingredients into dry. Stir with a spatula until the flour is just moistened. Stop while it’s still lumpy; overmixing makes tough muffins.

Add mix-ins: Toss berries or chips with a spoonful of flour to help suspend them, then fold into the batter with minimal strokes.

Fill the cups: Divide batter among the 12 liners, filling each almost to the top for big domes. Sprinkle with coarse sugar if you like.

Bake high, then lower: Bake at 425°F (220°C) for 5 minutes. Without opening the oven, reduce heat to 350°F (175°C) and bake another 12–15 minutes, until the tops are golden and a toothpick comes out with a few moist crumbs.

Cool smart: Let muffins sit in the pan for 5 minutes, then move to a rack. This keeps the bottoms from steaming and getting soggy.

Serve: Enjoy warm, or let cool completely before storing. The flavor is even better after an hour as the crumb sets.