Preheat oven to 375 degrees F. Line muffin tins with paper or foil cups.
Mash bananas in a medium bowl, leaving some small chunks. Stir in egg, sour cream and melted butter. In a large bowl combine the sugar, flour, baking soda salt, and espresso powder.
In a small, microwave-safe bowl, melt chocolate baking bar in the microwave on high power for 45 seconds. Stir. If un-melted pieces remain continue microwaving and stirring the chocolate in ten-second intervals until it is all melted and smooth; set aside.
Pour the banana mixture into the dry ingredients and stir just to combine. The batter should be lumpy. Stir in melted chocolate, 1/2 cup of mini chocolate chips, and vanilla. Divide batter among the prepared muffin cups and sprinkle each with the additional 1/4 cup mini chocolate chips for the topping.
Bake in preheated oven for 15 to 20 minutes. Muffins are done when the tops of the muffins look dry and cracked. Because of the melted chocolate chips in the muffins the traditional toothpick test may not be reliable.