Mix flour, baking powder and sugar in a bowl. Whisk egg, milk and oil with a whisk. Add the flour mixture, stir briefly
peel banana, halve lengthwise and cut into 2-3 mm thick slices
Roughly chop the walnuts. Fold banana and walnuts into the dough
Grease glasses and fill up to approx. 1/3 with dough. Bake in the preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: level 2) for approx. 35 minutes (test with sticks!)
In the meantime, bring a pot of water to a boil, place 6 suitable lids and preserving rings in it, boil for about 2 minutes, remove from the pot and let it drain on a tea towel
Remove the jars and close them directly with preserving rings, lids and clips. Allow to cool down
Cake can be kept unopened for at least 4 weeks