• Preheat the oven to 350 degrees F. Prepare a 9x13 cake pan with a generous spray of cooking spray. Set aside.

Make The Chocolate Cake

• In a large bowl, add the chocolate cake mix, large eggs, whole milk, oil, and vanilla extract. Beat with an electric mixer, on medium or medium high speed, for 2 minutes or until it's thoroughly combined.
  1 box (15.25 oz) chocolate cake mix, 3 large eggs, 1 cup whole milk, 1/2 cup vegetable oil, 1 teaspoon vanilla extract
• Pour the cake batter into the prepared cake pan. Bake for 25-35 minutes or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely inside the pan on a wire cooling rack.
• Once cooled, cover the cake pan with aluminum foil and freeze the cake for 30-60 minutes to allow the cake to harden slightly; this will make it easier to cut in half.

Make The Marshmallow Filling

• In a large mixing bowl, add the marshmallow fluff, butter, powdered sugar, heavy cream, vanilla extract, and salt. Beat on medium speed, increase speed as needed, until smooth, fluffy, and combined well. Reserve 1/2 cup for the swirls on top of the cake, set it aside.
  1 jar (13 oz) marshmallow fluff, 1 cup (2 sticks) butter, 3 cups powdered sugar, 1/4 cup heavy cream, 2 teaspoons vanilla extract, ¼ teaspoon kosher salt or sea salt

Assemble The Hostess Cake

• Remove the cake from the freezer. Invert the cake onto a large cutting board so it comes out of the cake pan. Cut the cake in half, lengthwise, so you end up with two thinner pieces of cake.
• Lay the bottom half of the cake back into the cake pan, and spread the marshmallow filling evenly over it. Then, cover the filling with the top half of the cake.
• Cover the pan with plastic wrap, foil, or a lid and refrigerate for 30 minutes.

Chocolate Ganache Frosting & Swirls

• Warm the heavy cream in a saucepan over medium heat, while gently whisking it until it starts to steam, and small bubbles form around the edges. Do not let it boil.
  1 cup heavy cream
• Place the chocolate chips into a heatproof bowl. Pour the hot cream over the chocolate and let it sit for 3 minutes. Gently whisk or stir the mixture until smooth and glossy.
  1 bag (12 oz) semi-sweet chocolate chips
• Add in the softened butter, vanilla extract, and pinch of kosher salt and mix together until everything is fully combined.
  2 tablespoons butter, 1 teaspoon vanilla extract
• Let the ganache cool for 15 minutes at room temperature, stirring occasionally, until it thickens into a spreadable consistency.
• Remove the cake from the fridge. Use an offset spatula or rubber spatula to spread the chocolate ganache frosting over top of the cake.
• Place the reserved white marshmallow filling into a piping bag with a thin tip, or a sandwich size Ziploc bag with the corner snipped, and pipe loops over top of the cake to resemble the classic Hostess Cupcakes.
• Slice and enjoy!