Ingredients

• 1 stick unsalted butter, melted and cooled (or 1/2 cup vegetable oil), plus more for the pan
• 1 1/4 cups all-purpose flour
• 3/4 cup granulated sugar
• 1 teaspoon baking powder
• 1 teaspoon salt
• 1/2 teaspoon baking soda
• 1/2 teaspoon ground cinnamon
• 1/2 teaspoon grated nutmeg
• 1 cup semisweet chocolate chips
• 2 large eggs
• 1/2 cup plain yogurt or sour cream
• 1 teaspoon vanilla extract
• 1 teaspoon grated orange zest (optional)
• 1 cup mashed banana
• 1 cup confectioners' sugar
• 2 tablespoons unsweetened cocoa powder
• 2 tablespoons milk
• 1/4 teaspoon vanilla extract
• Pinch of salt