Ingredients

• 1 stick (8 tablespoons) unsalted butter, melted, plus more for greasing
• 1 1/4 cups all-purpose flour, plus more for dusting the pan
• 1/2 teaspoon baking powder
• 1/2 teaspoon baking soda
• 1/2 teaspoon kosher salt
• 1/2 teaspoon ground cinnamon
• 1/4 teaspoon ground allspice
• 1/4 teaspoon ground ginger
• 1/2 cup granulated sugar
• 1/2 cup light brown sugar
• 1/2 cup mashed banana (from about 2 bananas) 
• 1/4 cup sour cream
• 2 large eggs
• 1 teaspoon pure vanilla extract
• 2 chocolate-coated vanilla ice cream bars, such as Klondike
• Confectioners' sugar, for dusting