On a saucepan on low heat, melt the butter and chocolate together. Let it cool.
Sift together the flour, cornstarch, the 120g of sugar and the baking soda. Add the eggs and mix well. Add the cool chocolate and butter mixture, and mix.
Mix together the bananas, the yoghurt, the tablespoon sugar, and the vanilla extract. Then mix it with the rest!
In an oiled mould, put the dough. Then put in the oven and cook at 180°C for 50 minutes-1 hour, or until a knife blade jabbed through the cake comes up clannish.
The cake will significantly harden as it cools--do leave it to cool before deciding it's not cooked!
Notes
The yoghurt can probably be replaced by 3 tablespoons sour cream, but you'll have to adjust the amount of flour and sugar to have a correct taste! (not tried this myself). The dough should be a little liquid, but not overly so (enough to ponderously flow into the mould from the mixing bowl, but not to run like a waterfall!) ; don't hesitate to adjust flour/sugar if it flows too much.
Recipe by Aliette de Bodard at https://www.aliettedebodard.com/recipes/banana-chocolate-cake/