1. Preheat oven to 180ºC. Prepare 12 muffins cups.
2. Make the banana muffins by combining the bananas, sugar, egg and melted butter in a large bowl. Add the flour and salt, and mix until just combined.
3. Spoon the batter into the muffin cups.
4. In a small bowl, mix the brown sugar, flour and cinnamon. Add in the butter and mix together. Rub between your finger tips until it forms crumbs. Put on top of muffin mixture in cups, and then place into the over.
5. Bake for 18-20 minutes, until cooked through.