Heat the oven to 160 degrees and spray a non stick muffin pan with cooking spray.
In a larger bowl add all the wet ingredients: eggs, vanilla essence, melted butter, mashed banana and yogurt.
In another smaller bowl add all the dry ingredients: flour, sugar, baking powder, cocoa, espresso and salt.
Using an electric mixer (or some muscle) mix the wet ingredients until well combined.
Add the dry ingredients into the wet ingredients one part at a time until all the ingredients are in a smooth batter. Fold in the chocolate chips reserving some for the top.
Pour the batter into the muffin tray cups filling each one. Use the reserved chocolate chips to sprinkle on each one before baking on the middle rack of the oven for 25 minutes.
When the muffins have risen in size and a toothpick comes out clean take them out and let them cool for 5 minutes before transferring them to cool further on a cooling rack.
Store in a sealed container or jar. Enjoy.