- Pre heat oven to 170°C 
- Stick your chocolate in the freezer before you begin.  
- In a large bowl, smush your bananas with a fork really well.  
- Add the sugar, egg, oil and vanilla and mix well with a whisk. 
- Sift the flour, cocoa powder, baking soda and salt into the wet ingredients. Fold the dry ingredients into the wet. 
- Get the chocolate out of the freezer and use a knife to chop half the bar into small pieces. Add these to the batter.   
- Line your muffin tray and fill each until 3/4th full.  
- Chop the rest of your chocolate into large chunks and top each muffin with a few pieces. 
- Bake for 20-24 minutes. Test with a toothpick to see if they're done. Alternatively, you could press the tops - they should bounce back when done.