Set the oven to 350°F (175°C). Grease and flour two 8-inch round cake pans or line with parchment paper.
In a bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon.
In a large bowl, cream softened butter and sugar until fluffy. Beat in eggs one at a time, then stir in vanilla. Mix in mashed bananas until smooth.
Add the dry mixture and buttermilk in alternating additions, beginning and ending with the flour. Stir until just combined—don’t overmix.
Divide the batter between pans and smooth the tops. Bake for 25–30 minutes, or until a toothpick comes out clean. Cool in pans for 10 minutes, then transfer to wire racks to cool completely.
Beat softened cream cheese and butter until smooth. Add powdered sugar gradually on low speed. Mix in vanilla and salt, then beat on medium-high for 2–3 minutes until fluffy.
Place one cake layer on a serving plate. Spread frosting over the top, add the second layer, and frost the top and sides. Decorate with a spatula or piping bag as desired.