Preheat the oven to 325 degrees F. Spray a 12-cup muffin pan with nonstick cooking spray.
Puree the bananas in a blender. (This will yield much smoother muffins than simply mashing them.)
In a medium bowl, stir together the banana, Splenda, melted butter, yogurt, vanilla extract, and egg.
In a separate bowl, whisk together the flours, instant espresso powder (if using), baking soda and salt.
Make a well in the middle of the dry ingredients. Pour the wet ingredients into the well and stir just until combined. Fold in the chocolate chips (if using).
Fill each muffin cup about ¾ full.
Bake in the center of the oven for 20-25 minutes, until a toothpick inserted in the center of the muffin comes out clean.
Move the muffin pan to a cooling rack, cool for 10-15 minutes. Remove muffins from the pan and let them finish cooling on the cooling rack.
Muffins can be stored in an airtight container with a paper towel inside for up to 3 days.