Preheat oven to 375 degrees. Line a standard size muffin tin with 12 cups.
In a medium sized bowl, whisk together the flour, sugar, cocoa, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer, combine the banana puree, yogurt (or sour cream), egg, and vanilla. Beat on low until combined. Add the melted/cooled butter, mix on low for 10 seconds.
With mixing speed on low, slowly incorporate the dry ingredients into the wet mixture. Do not overmix. Remove bowl from stand and fold in chocolate chips (if desired).
Using a cookie scoop, fill each muffin tin ¾ of the way full. Place in oven and bake for 18-24 minutes, or until a toothpick inserted into the center comes out clean.
Remove from oven and allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.