In medium bowl, combine the sugar, oil, eggs and stir with wire whisk. Add in bananas and vanilla, whisk until combined.
Remove muffins from pan to a cooling rack.
For the cinnamon & sugar topping : Combine the sugar and cinnamon in a small bowl. Put melted butter in a separate small bowl. Dip muffin tops into melted butter and then into cinnamon-sugar. Serve warm or let cool completely.
Feel free to also use a handheld blender or a stand mixer. I prefer to make these with just a wire whisk but a blender can be used just as well.
Readers have commented and said that they don't even do the cinnamon & sugar topping and the muffins are still delicious. Some have even reduced the sugar in the cinnamon & sugar topping.
Check the muffins to make sure that the center is still not wet looking. It needs to be cooked completely so that the muffins don't sink down in the middle. If you're worried, simply use a toothpick and make sure it comes out with moist/dry crumbs on it, NOT wet batter.