• Preheat the oven to 350°F. With a wooden spoon, mix butter into the mashed bananas in a large mixing bowl.
• Mix in the sugar, egg, espresso and vanilla.
• Sift in the baking soda and and the salt over the mixture and mix in.
• Sift the flour mix until it is just incorporated.
• Pour mixture into a prepared muffin tin.
• Bake for 25-30 minutes.
• Check for doneness with a toothpick inserted into the center of a muffin. If it comes out clean, it's done.
• Cool on a rack.