Prep the pan and oven. Heat oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners, or grease lightly. The higher starting temp helps create a domed top.

Mix dry ingredients. In a large bowl, whisk flour, sugar, baking powder, baking soda, salt, and cinnamon until well combined. This prevents clumps later.

Whisk the wet ingredients. In a separate bowl or large measuring cup, whisk buttermilk, oil, eggs, vanilla, and lime zest until smooth.

Combine gently. Pour wet ingredients into the dry. Stir with a spatula just until a few streaks of flour remain. Do not overmix—a lumpy batter is fine.

Fold in the fruit. Add mango, pineapple, and coconut (if using). Gently fold to distribute. If the fruit is very juicy, pat it dry first so the batter stays balanced.

Fill the cups. Divide batter among the muffin cups, filling them almost to the top. Sprinkle with turbinado sugar for a crunchy lid if you like.

Bake high, then lower. Bake at 400°F (200°C) for 7 minutes, then reduce heat to 350°F (175°C) without opening the oven. Continue baking 10–12 minutes, until tops are golden and a toothpick comes out with a few moist crumbs.

Cool smartly. Let muffins cool in the pan for 5 minutes, then transfer to a rack. This prevents soggy bottoms and keeps the crumb tender.

Finish and enjoy. Serve warm or at room temperature. A little butter or a drizzle of honey is never a bad idea.