Preheat oven to 375 degrees and line a muffin tin with paper liners.
Whisk together sugar, butter, and eggs until smooth. Beat in mashed bananas, vanilla and cinnamon.
In a separate bowl, whisk together flour, baking soda and salt. Add dry ingredients to wet and stir just until completely combined.
Divide batter evenly into muffin tin and bake 17-20 minutes, or until muffins are risen, golden and a tester inserted into muffin comes out dry.
Whisk together remaining cinnamon and sugar.
Dip each muffin top into the remaining melted butter and roll around in the cinnamon sugar to fully coat. Repeat with rest of muffins. Let cool completely on a wire rack (topping will harden slightly as it cools).