• Preheat the oven to 350° F and line two baking sheets with parchment paper.
• In a large mixing bowl, add (1) 15.25-ounce box chocolate cake mix, 2 large eggs, 1⁄3 cup vegetable oil, and 1 teaspoon vanilla extract. Use a hand mixer on medium speed to blend just until a thick, sticky dough forms, with no dry mix remaining. Scrape the bowl with a spatula to make sure everything is evenly combined.
• Scoop the dough using a 1-tablespoon cookie scoop and place each mound onto the prepared sheets, spacing them about 2 inches apart.
• Bake for 10-13 minutes, or until the tops look set and the edges are just firm. The centers should still look soft since they’ll continue to set as they cool. Remove from the oven and let the cookies rest on the baking sheet for 5 minutes before moving them to a cooling rack to cool completely.
• While the cookies cool, beat 1 cup (2 sticks) of unsalted butter until creamy. Add (1) 13-ounce jar marshmallow fluff, 2 cups powdered sugar, 1 teaspoon vanilla extract, 2 tablespoons heavy cream, and a pinch of kosher salt. Mix on medium speed until fluffy and smooth, about 2 minutes.

Marshmallow Filling

• Reserve ½ cup of the frosting and place it in a piping bag with a thin tip for later. Then transfer the remaining filling to another piping bag fitted with a round tip.

  Pipe a generous swirl on top of each cooled cookie, keeping about 1⁄4 inch from the edges. You may have some leftover frosting. Place the cookies in the fridge for 10 minutes to firm the tops.

Make the Ganache

• Heat 1⁄3 cup heavy cream in a microwave-safe bowl for 30-45 seconds or until warm but not boiling. Add 1 cup semisweet chocolate chips, 1 tablespoon softened butter, 1⁄2 teaspoon vanilla extract, and a pinch of kosher salt. Allow it to sit for 2 minutes, then whisk until glossy and smooth. Microwave for a few more seconds if the chocolate is still slightly lumpy. Don’t overheat the chocolate.
• Spoon or pipe warm ganache onto each cookie to fully cover the marshmallow frosting. Smooth out with the spoon or an offset spatula. Place the cookies back on the parchment and refrigerate for 10 minutes to help the chocolate set.
• Use your reserved marshmallow frosting in a piping bag to pipe the classic Hostess-style white loop swirl across the top of each cookie. Allow the swirl to set at room temperature for 15 minutes before serving. ENJOY!