Preheat oven to 350 F. Mash the bananas with a fork on a separate plate until no large clumps remain. You can also use an electric mixer or stand mixer with the paddle attachment to mash the bananas.
Add the mashed bananas, sugar, brown sugar, eggs, melted butter, milk, apple cider vinegar, and vanilla extract to a bowl and mix until well combined.
Fold in the gluten free flour, cinnamon, baking powder, and sea salt into the batter until no flour clumps remain.
Pour the batter evenly into three 6-inch cake pans that have been greased with oil. You can weigh the batter to make sure it’s distributed evenly or use a cookie scoop or measuring cup to evenly portion it out. Tap the pans on the counter a few times to release any air bubbles. Bake in the oven for 35 - 38 minutes until the edges of the cake are golden brown and you can insert a toothpick into the center and it comes out clean.
Let the cakes cool completely before icing. I like to pop them in the fridge so they have a chill to them before icing.
For the icing, beat the softened butter in a mixing bowl until light and fluffy. This can take a few minutes. Add 1 cup of the powdered sugar along with the maple syrup and cinnamon. Continue to mix to incorporate. Add the rest of the powdered sugar to the bowl in ½ cup increments. If needed, add 1 tablespoon of milk at a time to achieve your desired consistency.
When ready to assemble, shave the tops of the cakes so they are flat on top and not overly rounded. This will ensure nice even layers of icing. Place the first layer on a cake stand and then a turntable, if using. Spread a thin layer of icing on top of the cake layer with an uneven spatula or knife, pushing excess icing off the edges. Repeat this process with the second and third layers.
Use your knife or uneven spatula to spread a thin layer of icing around the sides of the cake. This is the crumb coat. Transfer the cake to the fridge or freezer for about 20 minutes so the icing can set.
Use the rest of the icing to apply a thick layer around the entire cake. Take a cake smoother and run it around the sides and top of the cake to create a smooth thick coat of icing.
To decorate, spoon a few spoonfuls of whipped cream on top of the cake, leaving some room around the edges as it will settle. Slice a banana in half vertically and lay the two slices on top of the cake followed by a dusting of crushed graham crackers. You can also use extra crushed graham crackers and press them around the bottom of the cake.
Use a thin sharp knife to slice the cake into pieces and enjoy!