Old-Fashioned Banana Cake

INGREDIENTS

• 6 tablespoons (3/4 stick) unsalted butter, at room temperature
• 1/2 teaspoon kosher salt
• 3/4 cup granulated sugar
• 1/2 cup vegetable oil
• 2 1/2 cups sifted confectioners' sugar (1/2 pound)
• 2 extra-large eggs, at room temperature
• 1/2 cup coarsely chopped walnuts (optional)
• 3 very ripe bananas, mashed
• 1/2 cup sour cream
• 1 teaspoon baking soda
• Grated zest of 1 orange
• 1 teaspoon pure vanilla extract
• 2 cups all-purpose flour
• Cream Cheese Frosting (recipe follows)
• Walnut halves, for decorating (optional)
• 1/2 cup light brown sugar, lightly packed
• 6 ounces cream cheese, at room temperature