Preheat your oven to 350°F (175°C). Prepare the Devil’s Food Cake mix according to the package instructions, pouring the batter into a greased 9x13-inch pan. Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean. Once baked, allow the cake to cool for about 10 minutes.

After the cake has cooled slightly, poke holes evenly throughout the surface using a large straw or the handle of a wooden spoon.

In a medium mixing bowl, combine the sweetened condensed milk, semi-sweet chocolate chips, and 1/3 cup of heavy cream. Whisk together until smooth and creamy—this should take about 2–3 minutes. Pour this mixture over the poked cake, making sure to fill all the holes generously. Allow it to soak in for 15–20 minutes.

In a separate large bowl, pour the remaining 1 1/2 cups of heavy cream. Using an electric mixer, whip the cream on medium-high speed until stiff peaks form, which should take around 3–5 minutes. Gradually fold in the marshmallow fluff until fully combined.

Spread the fluffy marshmallow cream over the soaked cake evenly using a spatula. Drizzle melted chocolate ganache over the top.

Cover the cake with plastic wrap or aluminum foil and refrigerate for at least 2 hours. Once chilled, slice into squares and serve cold.