Preheat oven to 375F (190C) and line 8 wells of a 12-count muffin tin with paper liners, alternating wells. (This recipe makes 15 muffins so we will bake in 2 batches, filling only half of the tin with each batch.) Set aside.

In a medium-sized mixing bowl, whisk together mashed banana, sour cream, butter, egg, and vanilla extract. Set aside.

Add the cocoa powder to a separate, heatproof bowl or glass measuring cup. Add hot water and whisk until smooth.

Carefully pour cocoa mixture into banana mixture and whisk until completely combined. Set aside.

In a separate, large mixing bowl, whisk together flour, sugar, baking soda and salt.

Pour wet mixture into the dry ingredients and use a spatula to gently fold ingredients together. When batter is about halfway combined, add chocolate chips and continue to gently fold together until all ingredients are just combined, being careful to not overmix.

Scoop batter into prepared muffin tin, filling each liner ¾ of the way full. Sprinkle each muffin top with a few additional chocolate chips, if desired. Remember you will only fill 7 or 8 liners this time, leave the remaining batter on the counter, covered with a towel or plastic wrap, to rest while the first batch bakes (this resting period will actually encourage even taller muffin tops for the second batch).

Transfer to center rack of 375F (190C) preheated oven and bake for 17-19 minutes, until a toothpick inserted into the center of the muffin comes out clean or with a few fudgy crumbs.

Allow muffins to cool in muffin tin for 5-8 minutes before removing to a cooling rack to finish cooling. Once the muffin tin has cooled completely, proceed with baking your second batch of muffins.