Preheat the oven to 350 degrees F.
In a large bowl, mix the flour, sugar, baking powder and salt.
In a separate mixing bowl, combine the mashed bananas, egg, melted butter and milk until incorporated.
Stir the banana mixture into the dry ingredients just until blended (but do not over-mix as it could result in a tough muffin). Stir in the chopped Mexican chocolate.
Divide the batter among the prepared muffin cups, filling each about 3/4 full.
Bake the muffins until the tops are pale golden and a tester inserted into the center comes out with some melted chocolate attached but no crumbs, about 30 minutes.
Transfer the muffins to rack to cool for 15 minutes. Then remove from the muffin tins.
The muffins can be stored in an airtight container for up to 4 days.