Preheat oven to 375° F. Grease or line a standard muffin pan with paper baking cups.
Make the Muffins
In medium bowl, whisk together ⅔ cup sugar, oil and eggs until combined.
Add mashed bananas and vanilla and stir together.
Add in remaining muffin ingredients and stir until just combined. Be careful not to overmix the batter.
Divide the batter among the baking cups.
Bake 18 minutes or until wooden pick inserted in center comes out clean.
Remove muffins from pan and allow to cool on a wire rack.
Make the Topping
While the muffins are cooling, melt the butter. Stir together ¼ cup sugar and ½ teaspoon cinnamon in small bowl.
With a pastry brush, brush the muffins tops with the melted butter. Using a tablespoon, sprinkle the cinnamon-sugar mixture over the tops of the muffins.
Be sure to use ripe bananas to get the best results. Under ripe bananas don’t add enough sweetness and overripe bananas can make the muffins gooey.
When adding the dry ingredients to the wet ingredients, be careful to not overmix the batter. Overmixing can lead to muffins that are tough.
Use a large scoop to fill the muffin cups with batter. This will keep the muffins similar in size and ensure even baking time.
Test the muffins for doneness at about 16 minutes. Oven temperatures can vary and while I bake the muffins for 18 minutes, you may need more or less baking time.
Remove the muffins from the pan to finish cooling on a wire rack. If you leave the muffins in the pan to cool, the bottoms of the muffins may become soggy.
Using a pastry brush to brush the melted butter on the tops of the muffins allows for even coverage without much mess.