Banana Buttermilk Waffles

Name: Banana Buttermilk Waffles
Ingredients: 2 cups all-purpose flour; 3 tablespoons granulated sugar; 2 teaspoons baking powder; ¼ teaspoon fine salt; 2 large eggs; 1¾ cups buttermilk, at room temperature; 1 very ripe banana, mashed, plus 1 banana, for serving (optional); 4 tablespoons unsalted butter, melted and cooled, plus more for the waffle iron; 1 teaspoon pure vanilla extract; ½ cup heavy cream, very cold; 2 tablespoons confectioners' sugar; ½ cup toasted walnuts, broken into small pieces; Maple syrup, for serving