Preheat oven to 375F and spray an 8x8 cooking dish well with cooking spray. Set aside.
Mash your bananas well in a bowl until no lumps remain (I use a potato masher). Add egg, yogurt, vanilla, and oil and mix until smooth.
In another bowl, combine flour, sugar, baking soda, baking powder, salt, and cinnamon and whisk together.
Add dry ingredients to wet ingredients and mix until you have a smooth batter.
Pour into pan (if desired, top with sliced banana, chocolate chips, nuts, etc) and bake 28-30 minutes, until lightly browned and a toothpick inserted comes out clean.
Let cool for 10 minutes before slicing into 12 squares. Enjoy!
Store leftovers in an airtight container in the fridge for up to 4 days. Pro tip: pop them in the microwave for a few seconds before enjoying!