Preheat oven to 425 degrees. Spray muffin pan with non stick cooking spray. Combine flour, soda, brown sugar, and cinnamon in a medium bowl. Set aside.
Smash bananas in a bowl or 4 cup measuring cup. Whisk together bananas, oil, milk, eggs, salt, and vanilla.
Make a well in the center of the dry ingredients. Pour in wet ingredients and lightly mix. Until lumpy. Do not overmix or muffins will be heavy and dense. Add nuts at this time if you want.
Using half cup measuring cup, put muffin mix in muffin pan. Divide accordingly. Once you are ready to bake, place in oven and lower oven temp to 375 degrees. Bake for twenty minutes or until toothpick inserted in center of muffin comes out clean.