Prep your oven and pan. Position a rack in the center of the oven and preheat the oven to 400°F. Line a muffin pan with six paper liners.
Mix the dry ingredients. In another medium bowl, whisk together the flour, baking soda, baking powder, and salt.
Make the banana mixture. In a medium bowl, use a fork to mash the bananas, adding the sour cream and vanilla and stirring them into the mixture as you do. The mixture will be thick and chunky, but that's totally okay, I promise!
Beat the butter and sugar, then add the egg. In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and sugar. Beat on medium until light, fluffy, and doubled in volume, 2 to 3 minutes, using a rubber spatula to scrape down the bottom and sides of the bowl as necessary. Reduce the mixer to low and add the egg, mixing until combined. Scrape down the bottom and sides of the bowl.
Add the dry ingredients, banana mixture, and chocolate chips. With the mixer on low, add the dry ingredients in two equal parts, adding the second half only when the first half has been just combined. Add the banana mixture all at once and beat on low until just combined. Add the chocolate chips and beat until evenly distributed. Scrape down the bottom and sides of the bowl once more, and beat on low for an additional 30 seconds.
Assemble the muffins. Use a 1-Tablespoon or 3-Tablespoon cookie dough scoop to fill each lined cavity with 6 scant tablespoons of the batter. Sprinkle the top of each muffin, aiming for the batter and avoiding the pan, with the chocolate chips for garnish. Pour warm water into the empty cavities of the muffin tin, filling them at least ⅔-rds of the way up. Bake the muffins. Bake for 22 to 24 minutes, or until the muffins are domed and golden brown around the edges. A skewer inserted into the center of a muffin should come out with few crumbs attached. Cool in the muffin tin on a wire rack for 10 minutes, then turn the muffins out onto the rack.
Serve and store. Serve warm or at room temperature. The muffins can be wrapped individually in plastic wrap and stored at room temperature for up to 2 days.