Malted Milk Sandwich Cookies

Name: Malted Milk Sandwich Cookies
Ingredients: For the cookies:; 4 cups all-purpose flour; 3/4 cup malt powder; 2 teaspoons baking powder; 1/2 teaspoon baking soda; 1/2 teaspoon salt; 1 cup (2 sticks) unsalted butter, at room temperature; 1 cup firmly packed dark brown sugar; 1 cup granulated sugar; 2 eggs; 1/3 cup sour cream; 2 teaspoons pure vanilla extract; For the vanilla filling:; 5 ounces vegetable shortening, at room temperature; 4 tablespoons (1/2 stick) unsalted butter, cut into small chunks, at room temperature; 3 1/4 cups confectioners’ sugar, sifted; 1 tablespoon pure vanilla extract; 1 teaspoon light rum