Caramel Walnut Upside Down Banana Cake

Ingredients

• nonstick vegetable oil spray
• 1/2 cup (1 stick) unsalted butter
• 1 cup (packed) golden brown sugar
• 3 tablespoons dark corn syrup
• 3/4 cup walnut halves or pieces
• 1 3/4 cups cake flour
• 1 teaspoon baking powder
• 3/4 teaspoon baking soda
• 1/2 teaspoon salt
• 1/2 cup (1 stick) unsalted butter, room temperature
• 1/2 cup sugar
• 1/2 cup (packed) golden brown sugar
• 2 large eggs
• 1 cup mashed very ripe bananas (2 to 3 large)
• 3 tablespoons sour cream
• 1 tablespoon dark rum
• 1 teaspoon vanilla extract
• whipped cream or vanilla ice cream