Start by working on your banana bar batter. Mash up the bananas and add them to a bowl with the oil, sugar, and eggs. Whisk until combined.
Whisk in the flour, baking powder, baking soda, salt, and cinnamon. If you're concerned about lumps, you can pour the liquid into a large bowl that already has the dry ingredients in it.
Fold in the walnuts or pecans if choose to add them. If you toasted them prior to adding them to the bars, they would taste even more amazing.
Once your batter is ready, pour into a greased pan and bake as directed. Allow the bars to cool, so the cream cheese frosting doesn't melt when you spread it on.