Preheat oven to 350°F. Grease a 13x9 inch pan with cooking spray.
In a large bowl, whisk together the flour, baking soda, and salt.
In another large bowl, beat the sugar and ½ cup butter on high until fluffy, about 2–3 minutes.
Add the eggs, vanilla, and bananas, and beat until blended. Scrape down the sides of the bowl with a spatula, if needed.
Add half of the flour mixture and half of the buttermilk. Stir gently with a spatula until incorporated.
Repeat with the remaining flour mixture and buttermilk. Stir in the walnuts.
Pour the batter into the greased 13x9 inch pan. Bake for 20 minutes, then put a piece of aluminum foil over the top to prevent the cake from browning too much. Bake for an additional 15 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool completely before spreading on the frosting.