Melt the chocolate. Cut into pieces, put in a smaller pot; put a bit of water in a larger pot, and the smaller pot into the larger pot. Bring to boil slowly, stop when the chocolate is melted.
Mix all the dry ingredients (flour, sugar, cacao powder, vanilla, salt).
Mush the banana with a fork. Put the butter in a cup and microwave on low power until melted.
To the melted chocolate, add butter, aquafaba, ½ cup of oat milk or water. Slowly add in dry ingredients, mixing with a spatula; add the rest of the milk.
Preheat the oven to 175 Celsius. In the meantime, put the mixture into a prepared oven pan.
Bake for 35-40 minutes, until the top is no longer shiny and the fork when inserted does not have marks.
Let cool for 10+ minutes before cutting.