Hot Cocoa Cupcakes

Name: Hot Cocoa Cupcakes
Ingredients: 1/2 cup buttermilk; 2 large eggs; 12 paper cupcake liners; 1 1/4 cups all-purpose flour; 1 cup granulated sugar; 1/3 cup unsweetened cocoa powder; 1 teaspoon baking powder; 1/4 teaspoon baking soda; 1/4 teaspoon kosher salt; 1/4 cup hot chocolate mix without marshmallows (one 1.38-ounce packet); 1/2 cup hot water; 1/3 cup canola or vegetable oil; 2 teaspoons vanilla extract; 8 tablespoons (1 stick) unsalted butter; 3 cups powdered sugar; 7 ounces marshmallow fluff (about 2 cups); 1 teaspoon vanilla extract; Hot chocolate mix without marshmallows (optional); Marshmallow bits or mini marshmallows (optional)