• Preheat oven to 350F (175C) and spray a 9x13" (approximately 23x33cm) pan with baking spray or lightly grease and flour.  Set aside.
• Combine butter and sugars in the bowl of a stand mixer (or in a large bowl and use an electric mixer). Beat until well-combined and light and fluffy.
  ¾ cup unsalted butter, 1 cup light or dark brown sugar, ½ cup sugar
• Add oil and beat to combine.  
  ¾ cup avocado, canola or vegetable oil
• Stir in mashed bananas.
  2 cups well-mashed ripe bananas
• Add eggs, buttermilk, and vanilla extract and stir until well-combined.
  2 large eggs, ¼ cup buttermilk, 1 Tablespoon vanilla extract
• In a separate bowl, stir together flour, baking soda, baking powder, and salt.
  2 ½ cups all-purpose flour, 1 teaspoons baking soda, 1 teaspoons baking powder, 1 teaspoon salt
• Gradually add to wet ingredients until completely combined.  
• Spread batter into prepared pan and bake on 350F (175C) for 45-55 minutes (see note 1) or until a toothpick inserted in the center comes out mostly clean with few moist crumbs.  Allow to cool completely before covering with frosting.

Silky Cream Cheese Frosting

• Combine cream cheese, butter, vanilla extract, and salt in a large bowl and beat with an electric mixer until creamy.
  8 oz cream cheese, ½ cup unsalted butter, 1 teaspoon vanilla extract, ½ teaspoon salt
• With mixer on low-speed, gradually add powdered sugar until completely combined.
  2 ½ cups powdered sugar
• Add heavy cream and gradually increase speed to high. Beat on high for 30 seconds.  
  1 Tablespoon heavy cream
• Spread over completely cooled banana cake. Top with chopped walnuts (if using).  Slice and serve!
  ½ cup chopped walnuts