Preheat the oven to 350° F. Line three 8-inch cake pans with parchment and spray with non-stick spray.
In a large bowl, whisk together the gluten free flour, cornstarch, baking powder, baking soda, and salt. Set aside.
Combing the buttermilk and vanilla in a large measuring cup with a spout. Set aside.
Cream the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment on medium high speed until the mixture is smooth and creamy, 3-4 minutes. Add the eggs, one at a time, mixing well after each and scraping the bowl afterwards.
Add the mashed bananas and mix on low speed just until combined.
Add the dry ingredients in three parts, alternating with the wet ingredients in two parts, beginning and ending with the dry. Keep the mixer at the lowest speed, and mix with each addition just until the ingredients are combined. When everything has been added, scrape the bowl and mix on medium-low until smooth, about 30 seconds.
Fill each cake pan with ⅓ of the batter and smooth the tops with an offset spatula (it is a thicker batter). Bake for about 25-28 minutes, or until a toothpick inserted in the center comes out clean.
Let the cakes cool in the pans for 10 minutes before removing to a rack to cool completely.
For the buttercream:
In a stand mixer fitted with the paddle attachment, cream the butter. Start with the mixer on low and then gradually increase the speed. The butter should be lighter in color. This should take about one minute. Scrape the bowl.
Add the sugar slowly and mix on low until all the sugar is combined. When the sugar has been fully incorporated, add the vanilla and cinnamon and mix to combine. Increase the speed to medium and beat until light and fluffy. This should take at least 5 minutes. You will notice the change in texture and the color will also lighten. Scrape the bowl and beater and mix briefly (about 30 seconds) just to combine once again.
Stack the cakes with about ¾ cup frosting in between. Frost the top and sides of the cake as desired. Cake will keep in a covered container at room temperature for 3-4 days.