Take advantage of a prepackaged muffin mix to keep the ingredient list short and the banana flavor big for these cookies. You can use a football cookie cutter or just cut in a ball-shape pattern.
In a large bowl, stir together muffin mix, flour, and brown sugar. Add the melted and slightly cooled butter all at once. Using a fork, stir just till a soft dough forms. (If necessary, add enough milk to form dough.)
Turn dough out onto a lightly floured surface. Pat or lightly roll dough till 1/8- to 1/4-inch thick. Using a football-shaped cookie cutter (or desired shape cutter), cut out cookies. Place 2 inches apart on cookie sheets lightly coated with nonstick cooking spray.
Bake in a 350 degree F oven for 9 to 10 minutes or until cookies begin to brown. Remove cookies to a wire rack to cool completely.
Meanwhile, prepare filling. For butter cream filling: In a medium bowl, beat powdered sugar, the 1/2 cup softened butter, and vanilla with an electric mixer until creamy. Spread a scant tablespoon of filling on bottom of 1 cookie; put together with another cookie, bottom side down, to make a cookie sandwich. Repeat with remaining cookies and filling.
Place white or chocolate pieces and shortening in a small microwave-safe bowl. Microwave on Medium (50 percent) power for 1 to 2 minutes or until pieces are melted and smooth, stirring every 30 seconds. Cool slightly (about 5 minutes). Carefully transfer melted chocolate to a heavy resealable plastic bag. Cut off tip of one corner of bag. Pipe melted chocolate onto cookies to resemble laces on footballs. Makes 12, 2-1/2- to 3-inch sandwich cookies.