Preheat the oven to 425°F (220°C). Line every other muffin well of your muffin pan with paper liners (if using); otherwise thoroughly grease your muffin pan. In a large mixing bowl whisk together the brown sugar, cooled melted butter, sugar and vanilla until well blended. Add in the eggs and mix for 1 minute more or until thick and combined.
Then add the cocoa powder, mixing until smooth. Grab a spatula and mix in the mashed banana, sour cream, baking powder, baking soda and salt until completely combined.
Gently fold in the flour until barely combined. Pour in the milk and grab your whisk again, whisking to combine. Scrape the sides and bottom of the bowl to make sure the banana muffin batter is thoroughly blended. At this point you can let your muffin batter rest for 20 minutes for a higher muffin top and/or stash in the fridge until the following morning (see Notes below recipe).
Add 2/3 cup (114 g) chocolate chips to the muffin batter, stirring until evenly distributed throughout. Scoop the muffin batter into the prepared muffin tins, filling every other muffin liner at least 3/4 full. Jiggle the pan a little so that the muffin batter levels out in the cups (the more even it is here, the more even the muffin rise will be).
Sprinkle the reserved chocolate chips over the tops of the muffins. Bake for 8 minutes in the center of the oven and then reduce the heat to 350℉ (177℃) and bake for 8 minutes more. When done, the tops will be puffed up tall; they should also bounce back when gently pressed in the center.
Let cool in the muffin pan for about 10ish minutes, then use a butter knife to gently get the warm muffins out of the pan before transferring them to a wire rack to finish cooling.
If You Don’t Have Butter (read this!) You can use 1/3 cup (75 g) cooking oil. You can use any cooking oil. I tested them with olive oil and sunflower seed oil and both worked great. Avocado oil would also be a good option. …see, I wasn’t kidding when I said I recipe tested these a lot!! You can mix the batter for the double chocolate banana chocolate chip muffins and store it covered in an airtight container, in the fridge, overnight. The muffin batter will be quite firm* in the morning (that’s ok). Let it sit out for about 45 minutes (or more) until it’s closer to room temperature. Scoop and bake the muffins as outlined in the recipe instructions. The colder the muffin batter is, the more time it will take to bake. You may need to add up to 5 minutes of bake time (at the 350℉ (177℃) temperature) depending on how cold the batter is. Conversely, the muffins will rise *less* if the batter is cold. Check the center of the muffin for doneness by pressing your finger gently in the center of the tops of the muffins. If it leaves an indent, bake for a wee bit longer. Lining Your Muffin Pans If you don’t have paper muffin liners, make sure to butter your muffin pan and sift unsweetened cocoa powder over the top, shaking off the excess (or use non-stick baking spray). This will keep your muffins from sticking to the pan.