1. Preheat the oven to 375°F (190°C). Line a standard muffin tin with paper or foil liners and set aside.

Prepare the wet mixture:

1. In a medium-sized bowl, mash the bananas until mostly smooth, leaving a few small lumps for texture. Stir in the beaten egg, sour cream, melted butter, and vanilla extract. Mix until well combined.

Combine the dry ingredients:

1. In a large mixing bowl, whisk together the granulated sugar, all-purpose flour, baking soda, salt, and espresso powder until evenly blended.

Melt the chocolate:

1. Place the chopped dark chocolate bar into a microwave-safe bowl and heat on high power for 45 seconds. Stir thoroughly, then continue heating in 10-second intervals, stirring after each, until completely melted and smooth. Allow the chocolate to cool slightly.

Incorporate the ingredients:

1. Add the banana mixture to the bowl of dry ingredients. Stir gently until just combined; do not overmix. The batter will remain slightly lumpy. Fold in the melted chocolate, 1/2 cup of mini chocolate chips, and vanilla extract until evenly distributed.

Portion and bake:

1. Divide the batter evenly among the prepared muffin cups. Sprinkle the tops with the remaining 1/4 cup mini chocolate chips.
2. Bake the muffins in the preheated oven for 15–20 minutes, rotating the pan halfway through baking. Muffins are done when the tops appear set and slightly cracked. Due to the presence of melted chocolate, a toothpick test may be unreliable.

Cool and serve:

1. Remove the muffins from the oven and allow them to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.