Preheat oven to 400°.
Spray 18 regular-sized muffin tins with cooking spray; set aside.
Mash bananas on a plate with a fork; transfer to a medium-sized mixing bowl; set aside.
In a large mixing bowl, combine flour, brown sugar, baking powder, baking soda and salt.
In the medium-sized mixing bowl, combine bananas, half-and-half, egg, oil and vanilla.
Whisk until well-combined.
(There may still be pieces of banana. That’s okay).
Add wet ingredients to the dry ingredients and stir just until moistened.
Do not overmix the batter.
Fill each muffin cup about 1/3 full of batter.
Break each candy bar apart into the pre-lined rectangular pieces.
Place two pieces over top of the muffin batter in muffin cup.
(This works better using the 1.55-oz. chocolate bars since the X-large candy bars have larger pre-lined pieces.)
Top with batter until muffin cups are almost full.
Chop remaining candy bar pieces.
Sprinkle the chopped chocolate candy bar pieces over top of the muffin batter.
Bake muffins at 400° for about 15 minutes, or until a toothpick inserted in center comes out clean and muffins are set.
Cool muffins 5 minutes in muffin pans before removing muffins to a wire cooling rack to cool completely.