Preheat your oven to 180°C (160°C fan) or gas mark 4. Lightly oil a small baking dish or four individual ramekins to prevent sticking.
In a large mixing bowl, combine the oat flour, protein powder, baking powder, cinnamon, and mixed spice. Stir until evenly mixed.
In a separate bowl, mash the bananas until smooth, then whisk in the milk and beaten egg. Gradually add the wet ingredients to the dry ingredients, stirring until you have a smooth, cake batter-like consistency.
Spoon the mixture into your prepared dish(es). Swirl the peanut butter through the top using a knife for an attractive, marbled effect.
Bake for 20-25 minutes, or until the oats are puffed up and golden brown on top. A skewer should come out clean when inserted into the center.
Let the baked oats cool slightly before serving. Top with a dollop of yogurt, a handful of fresh berries, and an extra drizzle of peanut butter if desired.