Preheat the oven to 375°F. Line a 12-cup muffin pan with liners or lightly grease it.
Prepare the mango. If using frozen mango, let it thaw slightly and pat away excess moisture. Dice it into small pieces, about ¼–½ inch.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.
In a large bowl, whisk the melted butter and sugar together. Add the eggs and vanilla and whisk until smooth.
Stir in the milk.
Add the dry ingredients to the wet ingredients and gently fold together until they're just combined. Don't overmix—a few small lumps are perfectly fine.
Gently fold in the 1½ cups of mango.
Divide the batter evenly among the 12 muffin cups. They'll be fairly full.
If you'd like a slightly crunchy bakery-style top, sprinkle a little granulated sugar over each muffin.
Bake at 375°F for 18–22 minutes, or until the tops are golden and a toothpick inserted into the muffin portion comes out clean. Because there are juicy mango pieces inside, don't worry if the toothpick hits a little mango.
Let them cool in the pan for about 5–10 minutes, then transfer them to a cooling rack.