Preheat oven to 350 degrees. Grease a 9x9 cake pan and set aside. Note: if you are using a dark cake pan, reduce oven temperature to 325 degrees to prevent edges of cake from browning too much.
In a medium bowl, combine, flour, baking soda, ground cinnamon, and salt. Set aside.
In a bowl fitted for a stand mixer, beat butter until fluffy, about 1 minute. Add sugars and continue beating until creamy, about 3 minutes. Add eggs, one at a time until combined. Scrap down the sides of the bowl. Add vanilla and mashed bananas and combine.
Add 1/3 flour mixture to wet ingredients and combine. Add 3/4 cup of buttermilk to wet ingredients and combine. Repeat one more time. Finish mixing process by adding last 1/3 of flour mixture and combining. Do not overmix - a few lumps are okay.
Pour batter into prepared cake pan and smooth the top with a spatula. Bake for 45-50 minutes or until a toothpick comes out clean when inserted in the center. Remove cake from the oven and let it cool completely before topping with frosting.