Preheat the oven to 180°C (350°F). Grease or line a small baking pan.
In a mixing bowl, pour the hot milk over the oat flakes and let them soak for 10 minutes.
Add the mashed bananas, eggs, and vanillin to the soaked oats. Stir until well combined.
Mix in cocoa powder and baking powder until the batter is smooth.
Fold in chopped walnuts and chocolate chips if using.
Pour the mixture into the prepared baking pan and smooth the top.
Bake for 25–30 minutes, or until a toothpick inserted comes out with moist crumbs.
Let cool slightly before slicing and serving.