Preheat oven to 425°F and prepare a muffin tray with liners or by greasing it. *NOTE: this is not the temperature for the full baking time.
In a large mixing bowl, combine mashed bananas, plain Greek yogurt, oil, eggs and vanilla extract. Mix thoroughly.
In a separate mixing bowl, combine flour, sugar, baking powder, baking soda, cinnamon and salt.
Add the dry ingredients to the wet ingredients and stir in the espresso powder and chocolate chips.
Pour the batter into the muffin tins. Bake for 5 minutes and then reduce the temperature to 325°F and cook for another 10-12 minutes, or until an inserted toothpick comes out clean.