Using a whisk or an electric mixer, beat together the butter and sugar until it goes lighter in colour.
Add the egg and beat once again.
Add the banana. If you're using a whisk, it's best if you first purée the banana with a fork. If you're using an electric mixer, you can add the banana chopped up and let the mixer purée it.
Add the sourdough discard and beat to incorporate.
Add the four, baking powder and salt and mix them in with a spatula. Add the chopped chocolate and incorporate it.
Pour the batter into a rectangular loaf pan 22x11cm previously greased and floured or prepped with baking paper.
Bake the loaf in a preheated oven at 180°C/350°F for 1 hour or until you can insert a toothpick or knife at the centre and it comes out without any dough (it might have chocolate, though).
Unmould while warm.