Preheat the oven to 350 degrees F. Butter the bottom and sides of two (8-inch) round cake pans. Line the bottoms of both pans with parchment paper rounds.
Add cake flour, all-purpose flour, baking soda, baking powder, and fine sea salt to small bowl. Whisk to combine. Set aside.
Add the egg whites to stand mixer. Beat on high to soft peaks. Transfer egg whites to small bowl. Set aside.
Add granulated sugar and butter to stand mixer. Beat on medium for 2 minutes or until mixture is light and fluffy.
Add egg yolks to sugar mixture. Beat on low until combined.
Add mashed bananas to sugar mixture. Beat on low until combined.
Add buttermilk to sugar mixture. Beat on low until combined.
Remove bowl from stand mixer. Add flour mixture and chopped toasted pecans and fold in using a large spatula, just until combined. Avoid over mixing.
Fold in reserved egg whites.
Transfer cake batter to prepared cake pans. Bake cake layers in preheated oven for 25 to 30 minutes or until cakes are firm to the touch and a toothpick inserted in the center comes out clean.
Remove cake pans from oven and cool on wire racks for 15 minutes. Invert pans and remove cakes. Cool completely before frosting.