Peel the bananas and slice them into 1 centimeter / 0.5 inch thick slices.
In a jam pan, mix the banana slices, water, sugar and one lemon juice. Bring to a boil. Pour into a bowl. Grate the chocolate, add it to the bananas and mix until completely melted. Cover with a foil and keep in a cool place overnight.
On the next day, bring it to a boil while stirring continuously. Mix the jam very finely. Keep on a low flame for 5 minutes. Skim carefully.
Bring again to a boil. Check the jam by pouring a few drops on a cold plate or spoon. Immediately pour into some jars and close them.
I have done the pear and chocolate jam by the same author (Christine Ferber; i think her books are translated into english), and it was *very* yummy.
Turn the closed jars upside down while they are hot, and wait until they are cold to turn them back again. This way, and if your jars were clean, you'll be able to keep the jam for a long time.