• Preheat the oven to 350°F. Spray a 9x13-inch cake pan with nonstick cooking spray and/or line with parchment paper.

Devil's Food Chocolate Cake

• In a large bowl, add the devil's food cake mix, whole milk, vegetable oil, large eggs, and pure vanilla extract. Using an electric hand mixer, beat at medium speed for 2 minutes, until the batter looks smooth and glossy.

  ^{*Scrape down the sides and bottom of the bowl as needed with a rubber spatula.}
  1 box (15.25 oz) devil's food cake mix, 1 cup whole milk, ½ cup vegetable oil, 3 large eggs, 2 teaspoons pure vanilla extract
• Pour the chocolate cake batter evenly into the prepared cake pan and smooth out the top. Bake for 22-30 minutes, or until a toothpick inserted into the middle comes out clean or with a few crumbs.
• Remove the cake from the oven and let it cool for 20 minutes, then use the handle of a wooden spoon to poke holes, evenly in rows, all across the cake, spacing them about 1-inch apart.
• Let the cake cool completely before adding the filling.

Whipped Marshmallow Cream Filling

• In a large bowl, add the heavy whipping cream, powdered sugar, vanilla extract, and salt. Using an electric hand mixer, or hand mixer, beat at medium-high speed until stiff soft peaks form (which means the whipped cream should hold its shape but still look creamy and soft).
  1 cup heavy whipping cream, ½ cup powdered sugar, 1 teaspoon pure vanilla extract, ¼ teaspoon salt
• Add the marshmallow fluff to the whipped cream mixture and beat on low speed just until combined, smooth, and fluffy.
  1 jar (7 oz) marshmallow fluff
• Pipe or spoon the filling into the holes and over the top of the cooled cake. Then smooth it out with an offset spatula.

  ^{*I like to put some filling into a Ziploc bag with the corner snipped, and then use that to fill each hole, that way I know each hole is fully filled.}
• Cover the cake pan with a lid or plastic wrap, and refrigerate for at least 1 hour (or up to 6-8 hours if needed) so the cream filling can settle into the cake and firm slightly before frosting.

Chocolate Buttercream

• In a large mixing bowl, or the bowl of a stand mixer, beat the butter on medium speed for 2-3 minutes, until very smooth, creamy, and whipped.
  1 cup (2 sticks) butter
• Add the powdered sugar and cocoa powder, then beat on low speed just until the dry ingredients are incorporated.
  4 cups powdered sugar, ½ cup unsweetened cocoa powder
• Add in the heavy whipping cream, vanilla extract, and kosher salt. Beat on medium speed for 2-3 minutes, until the chocolate buttercream is smooth, fluffy, and easy to spread.

  ^{*If the frosting is still too stiff, add additional heavy whipping cream, 1 tablespoon at a time, until you reach the desired consistency.}
  ½ cup heavy whipping cream, 2 teaspoons pure vanilla extract, ¼ teaspoon salt
• Evenly spread the buttercream over the chilled cake, over the marshmallow layer, and spread it out using an offset spatula using light pressure.
• Use a fork or the tip of an offset spatula to make soft lengthwise lines through the chocolate buttercream, similar to the ridged chocolate top on a Devil's Food Zinger.
• Cover the cake and refrigerate for at least 1 hour, or up to 8 hours, before serving.
• Slice the cake chilled from the fridge, and let each slice sit at room temperature for about 5-10 minutes before serving, for the softest texture. Enjoy!