Preheat the oven to 325 (F).
Combine 6 tablespoons butter and 1 cup sugar in the bowl of a stand mixer fitted with a paddle attachment and cream together on medium-high for 2 to 3 minutes. Scrape down the sides of the bowl, add 1 egg, and mix on medium-high again for 2 to 3 minutes. Scrape down the sides of the bowl once more.
Stream in 1/2 cup buttermilk and 2 tablespoons grapeseed oil while the paddle swirls on low speed. Once the buttermilk and grapeseed oil has been added, increase the mixer to medium-high and paddle for 5 to 6 minutes until the mixture is practically white, completely homogenous, and twice the size of your original fluffy butter-and-sugar mixture. You're basically forcing too much liquid into an already fatty mixture that doesn't want to make room for it, so if it doesn't look right after 6 minutes, keep mixing. Stop the mixer and scrape down the sides of the bowl.
On very low speed, add 2 bananas and mix for 45 to 60 seconds to ensure all the bananas are broken apart.
Still on low speed, add 1 1/3 cups flour, 3/4 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/2 teaspoon kosher salt. Mix for 45 to 60 seconds, or just until your batter comes together and any remnants of dry ingredients have been incorporated. DO NOT OVERMIX or your cake will be tough and I will be sad. Scrape down the sides of the bowl.
Pam-spray te 3 cake pans and line with parchment. Using a rubber spatula, divide the batter into three equal portions and spread the cake batter in even layers across the three pans. Give the bottom of the cake pans a tap on the countertop to even out the layers and bake in the preheated oven for 25 to 30 minutes. At 25 minutes, gently poke the edge of the cake with your finger -- the cake should bounce back slightly and the center should no longer be jiggly. Leave the cake in the oven for an extra 3 to 5 minutes if the cake doesn't pass these tests. Take the cake out of the oven and cool on a wire rack.