Preheat oven to 350°F (175°C). Line a standard muffin tin with paper liners or lightly grease the cups.

In a large mixing bowl, whisk together flour, granulated sugar, baking powder, cinnamon, salt, and cream of tartar until evenly combined.

In a medium bowl, whisk together milk, egg, melted butter, and vanilla until smooth.

Pour the wet ingredients into the dry ingredients and gently fold until no large lumps remain. The batter will be slightly runny.

In a small bowl, combine the granulated sugar and cinnamon for the topping.

Fill each muffin cup about ⅔ full with batter. Sprinkle the cinnamon sugar mixture evenly over the tops of the batter, being careful to avoid the edges of the pan to prevent burning.

Bake for 20–24 minutes, or until a toothpick inserted into the center comes out with a few crumbs, but no wet batter.

Let the muffins cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.