Preheat oven to 350 degrees.
Prep sheet pan by lining it with parchment paper and spraying the paper and pan riser with pan spray or softened butter.
In a small/medium bowl combine dry ingredients- flour, baking powder, baking soda, salt, and cinnamon.
In another larger bowl, mix butter and sugar. You can do this with a stand mixer, hand mixer, or even by hand. It will be a good workout if you decide to do it by hand! Beat the butter and sugars until they are light and fluffy. Add in bananas and mix again. You can mush them beforehand, but it is so simple to throw them in and mix them. Next are eggs and vanilla. Blend this only until combined. Then add dry ingredients. Now, only blend these until they just come together, you don't want to overmix baked goods with whole wheat flour. Last is your buttermilk. Again, be careful not to overmix.
Pour batter into pan. Spread carefully, but don't stress about it being completely flat, it will do this as it bakes.
Now, bake until a toothpick inserted in the center comes clean. I like to underbake just a touch. Whole-grain baked goods can dry out quickly. It is good to underbake just a little.
Place on a cooling rack. Within a minute or 2, remove the riser.
Allow cooling before icing.