Preheat your oven to 375°F (190°C). Prepare a muffin pan with nonstick cooking spray or line with parchment paper or muffin liners.
In a medium bowl, whisk together the granulated sugar, vegetable oil, and eggs until well combined.
Add the mashed bananas and vanilla extract to the sugar mixture, whisking until smooth.
Stir in the all-purpose flour, baking soda, ground cinnamon, and salt using a rubber spatula, mixing just until combined and no streaks of flour remain in the batter.
Divide the batter evenly among the 12 muffin cups, filling each cup slightly over 3/4 full.
Bake for 16 to 21 minutes, or until a toothpick inserted into the center of the muffins comes out clean and the tops are high and rounded. The tops should not appear wet, and they should spring back when lightly pressed.
Remove the muffins from the oven and allow them to cool in the pan for 5 minutes before transferring them to a cooling rack. Let them cool enough to handle comfortably.
For the cinnamon-sugar topping, combine the granulated sugar and ground cinnamon in a small bowl. In a separate bowl, place the melted butter.
Dip the top of each muffin into the melted butter, then into the cinnamon-sugar mixture.
Serve the muffins warm, or let them cool completely on the cooling rack before enjoying.