Cook the Book: Sweet Potato Waffles

Name: Cook the Book: Sweet Potato Waffles
Ingredients: 8 ounces sweet potatoes, peeled and cubed; 1 cup whole milk; 2 ounces light brown sugar; 4 tablespoons unsalted butter, melted; 1 tablespoon orange zest, grated; 10 1/2 ounces all-purpose flour; 1 tablespoon baking powder; 1/2 teaspoon kosher salt; 6 large egg whites at room temperature