Mom's Whoopie Pies

Ingredients

• 1 cup sugar
• 1 egg
• ½ cup unsalted butter, softened
• 1 teaspoon vanilla extract
• ½ cup cocoa, measured then sifted
• 1 teaspoon baking soda
• 1/2 teaspoon salt
• 2 cups all purpose flour
• 1 cup buttermilk
• 3/4 cup unsalted butter, room temperature
• 1 cup confectioner’s sugar, sifted
• 1 1/2 cups prepared marshmallow
• 2 teaspoons vanilla extract
• ¼ teaspoon salt