Preheat oven to 350 degrees.
Line the pans: Line 2 mini-muffin pans with 18 mini-muffin liners.
Mix egg and sugar: Crack the egg into a large bowl, then add the brown sugar.
Whisk together: Add the vanilla extract and beat together until thick.
Mash the banana: Break the banana into pieces in a small bowl, then mash it well with a fork.
Add to egg mixture: Add the mashed banana and the oil to the egg-and-sugar mixture.
Sift the flour: Sift in the flour, baking powder, baking soda, salt and cinnamon. Add any of the mixture left in the strainer to the bowl, too. Stir together until just combined.
Fill the muffin liners: Spoon into the muffin liners (around two-thirds full). Then bake for 12-14 minutes until risen and firm.
Store in an airtight box for up to 3 days.