1. Mash the ripe bananas in a mixing bowl until mostly smooth.
2. Add the egg, sugar, milk, oil or melted butter, and vanilla. Whisk until combined.
3. In a separate bowl, mix the flour, baking powder, cinnamon, and salt.
4. Add the dry ingredients to the banana mixture and stir just until combined.
5. Lightly grease an 8-inch nonstick pan with a lid. Warm it over very low heat.
6. Pour the batter into the pan and spread evenly. Cover with the lid.
7. Cook over very low heat for about 20–25 minutes, checking near the end.
8. Test the center with a toothpick. If wet batter remains, cover and cook for another 3–5 minutes.
9. If the top is not fully set, carefully flip the cake and cook for another 3–5 minutes.
10. Rest for 5 minutes, transfer to a plate, and slice.