Grind the oatmeal finely into a powder.
Melt the dark chocolate and unsalted butter together in a double boiler.
Mash the banana in a bowl, then add the egg, melted chocolate, and almond milk and mix well.
Sift in the oat flour, cocoa powder, and baking powder, then mix until combined.
Pour the batter into a mold and bake in a preheated oven at 180°C for 20 minutes.