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.