1. Cook the pumpkin until soft. Cooking options include steaming or microwaving.
2. Once the pumpkin is cooked, transfer it to a container and combine with the banana, egg, lemon zest (optional), oat flour, and cinnamon. You can add any sweetener you prefer.
3. In another container, beat egg whites until stiff.
4. Combine the two mixtures and add some baking powder.
5. Grease a mold before pouring the mixture in.
6. Bake for 30 minutes at 180°C/350ºF.
7. Allow it to rest at room temperature.