Classic Whoopie Pies

INGREDIENTS

• Cakes:
• 1/2 cup butter
• 1 cup brown sugar, packed
• 1 teaspoon espresso powder, optional
• 1 teaspoon baking powder
• 1/2 teaspoon baking soda
• 3/4 teaspoon salt
• 1 teaspoon vanilla extract
• 1 large egg
• 1/2 cup Dutch-process cocoa
• 2 1/3 cups all-purpose flour
• 1 cup milk
• Filling
• 1/2 cup vegetable shortening
• 1/2 cup butter, room temperature
• 1 cup confectioners' sugar or glazing sugar
• 1 1/3 cups Marshmallow Fluff or marshmallow creme
• 1/4 teaspoon salt dissolved in 1 tablespoon water
• 1 1/2 teaspoons vanilla extract