S'mores Whoopie Pies

INGREDIENTS

• :
• 1½ cups graham flour
• ¾ cup all-purpose flour
• 1½ teaspoons baking powder
• ½ teaspoon salt
• 4 tablespoons unsalted butter, at room temperature
• 4 tablespoons vegetable shortening
• 1 cup (packed) dark brown sugar
• 2 large eggs
• ½ cup buttermilk
• 2 tablespoons milk
• 1 teaspoon baking soda
• 1 teaspoon white vinegar
• 1 teaspoon vanilla extract
• 1½ cups Marshmallow Fluff (or other prepared marshmallow cream, which will do in a pinch)
• 1¼ cups vegetable shortening
• 1 cup confectioners' sugar
• 1 tablespoon vanilla extract
• 8 ounces milk chocolate chips (or solid chocolate, finely chopped)
• ½ cup heavy cream