Preheat the oven to 350 degrees. Spray 3 6" cake pans with non stick spray and line the bottoms with parchment paper rounds.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt, then set aside.
2 ¼ cups (282 g) all-purpose flour, spooned and leveled , 1 ½ teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon salt
In a large bowl, cream the butter and sugar together with an electric mixer on high speed until it is light and fluffy, about 3-4 minutes.
10 tablespoon (140 g) unsalted butter, softened, 1 ½ cups (300 g) granulated white sugar
Next add in the eggs, egg yolks and vanilla and mix on medium-high speed until pale in color, smooth and slightly fluffy, about 1-2 minutes. Scrape the sides and bottom of the bowl with a spatula as necessary.
2 eggs, at room temperature, 2 egg yolks, at room temperature, 2 teaspoon vanilla
Then add in the mashed banana and combine on low speed.
1 cup (250 g) mashed banana
Add the dry ingredients and the buttermilk alternately to the butter mixture and mix on low then medium speed until combined and the batter is smooth.
½ cup (120 ml) buttermilk, at room temperature
Divide the batter evenly among the prepared cake pans.
Bake the cakes for 30-34 minutes, or until a cake tester/toothpick comes out clean from the centers.
Let the cakes cool in the pans for 2 minutes, then transfer them to a cooling rack to completely cool.