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