Mix the cocoa with the sugar in a small cup until it's well combined.
Mix in the yoghurt, until it's an even colour with no lumps or splotches. Should be quite dark.
Peel the banana. Halve it lengthways and then slice into about finger thickness pieces.
If this is in the colder months, then microwave the banana on full power for one minute.
If this is in the colder months, I would then - after the banana - heat the tortilla: I like to do so on an electric cooker at the lowest or second-lowest level for two to three minutes on each side. This also works for defrosting them nicely.
When the tortilla is warm and soft, spread it flat on a plate. Place the banana slices in the middle. If your banana was big, it might not fit. Use your own judgement.
Spoon the chocolate yoghurt ( healthy pud! ) on top of the banana. There might be a bit too much and you could have a little leftover. I"m sure you'll be able to find a place for it ;).
Make sure you've filled the tortilla up to an inch from the front - whichever side you'll be biting into.
Wrap up ( I fold one side over the filling, tuck the bottom in and fold the other side over)and DIG IN!