Combine and sift the ingredients marked ☆. Cut the chocolate, butter, and banana into pieces, put them in a heatproof container and microwave for 1:30 minutes, until the butter and chocolate has melted.
Mix the melted mixture well with a whisk. When it becomes smooth, add the beaten egg a little at a time and mix. Add the sugar and rum and mix some more.
Add the sifted ☆ ingredients and fold in using a rubber spatula. Pour it into the mould and lightly tap it a few times to get rid of any air bubbles. Put the walnuts on the surface of the mixture and bake for 30 minutes in an oven preheated to 170℃.
When a bamboo skewer poked through the middle comes out clean, it's done.
Story Behind this Recipe
I made a banana version of my original apple brownie recipe.
When you microwave the banana, it will melt and you'll get a smooth batter. If you'd prefer the banana to keep its texture, mash it with a fork instead of microwaving it, then add it to the mixture. If the chocolate you are using is not so sweet, add some more sugar.