1. Mash the bananas and put in a small heavy saucepan with the lemon juice, sugar and butter. Bring it to a simmer over medium heat and then turn to low. Simmer for 20 minutes, stirring every 5 minutes or so.
2. Turn off the heat and let it cool until it is cool enough to taste. Taste and then add the cinnamon and vanilla by 1/2 teaspoon-fuls until it has the taste you like. The jam will still be a little liquid at this point - don't worry!
3. Let the jam cool completely to room temperature - it will thicken up as it cools to about the consistency of apple butter.
Store in the fridge for a week or so - if it even lasts that long!