Preheat the oven to 375°F (190°C). Line a 6-cup muffin tin with paper liners or grease the cups.

In a medium bowl, whisk together flour, sugar, baking powder, baking soda, salt, and cinnamon until evenly combined.

In a separate bowl, whisk the melted (but not hot) butter with the egg, vanilla, and milk until smooth.

Pour the wet ingredients into the dry and gently fold with a spatula just until no large streaks of flour remain. The batter should be thick but spoonable. Do not overmix.

Divide the batter among the muffin cups (about 1/4 to 1/3 cup per muffin) so they’re about 3/4 full.

Optional: Combine 1 tablespoon sugar with 1/2 teaspoon ground cinnamon and sprinkle over each muffin.

Bake for 14–16 minutes, rotating the pan halfway through, until the tops are golden and a toothpick inserted into the center comes out with just a few moist crumbs.

Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to finish cooling.