Add the bananas and lemon juice to a preserving pan and toss together. Add the measurement water, then cook gently, uncovered, for 5 minutes, until the bananas are just beginning to soften.
Pour the sugar into the pan and heat gently, stirring from time to time, until dissolved. Bring to the boil, then boil for 3–5 minutes until very thick and the jam slowly drops from a spoon. As this jam thickens so quickly, test for a set with a spoon rather than a sugar thermometer or wrinkle test on a saucer.
Turn off the heat and stir in the chocolate. Ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Store in the refrigerator.
To serve, this jam can be spread over crepes and dusted with icing sugar.