Preheat oven to 350.
Cream together the butter and the brown sugar until light and fluffy. Add the egg and mashed bananas, and continue beating until smooth.
In another bowl, whisk together the flour, salt, baking soda, cinnamon and chocolate. Add this to the wet ingredients, along with the milk, and mix until just combined.
Spray a muffin tin with nonstick spray and fill up tin 3/4 of the way full with batter.
Bake muffins for about 20-22 minutes until golden.