Preheat the oven to 375°F and line a 12-cup muffin tin with paper liners or grease lightly with butter.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt until evenly combined.
In a separate bowl, whisk together the melted butter, maple syrup, milk, egg, and vanilla extract until smooth and fully incorporated.
Pour the wet ingredients into the dry ingredients and gently fold together with a spatula until just combined. The batter should be thick but don’t overmix; a few lumps are okay.
Divide the batter evenly among the prepared muffin cups, filling each about 2/3 full. Sprinkle the tops generously with maple sugar or coarse sugar.
Bake for 18–20 minutes, or until the muffins are golden brown and a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely. Serve warm or at room temperature.