Gluten-Free Whoopie Pies

Ingredients (17)

• 1 cup white rice flour
• 1/2 cup sweet rice flour
• 1/2 cup Dutch-process cocoa powder
• 1/2 cup tapioca starch
• 1 1/4 teaspoons baking soda
• 1 teaspoon salt
• 1/2 teaspoon xanthan gum
• 1/2 cup (8 tablespoons) unsalted butter, at room temperature
• 1 cup packed dark brown sugar
• 1 large egg
• 1 teaspoon pure vanilla extract
• 1 cup buttermilk
• 1/2 cup (8 tablespoons) unsalted butter, at room temperature
• 1/4 cup vegetable shortening
• 3/4 cup powdered sugar
• 1 jar (7 1/2 ounces) marshmallow creme
• 1 teaspoon pure vanilla extract