Preheat oven to 350ºF and spray a 9x13-inch baking dish with nonstick cooking spray.

Prepare the cake batter according to the directions on the box. Pour batter into prepared pan, smoothing out the batter. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Remove from the oven, and cool completely.

Combine all of the ingredients in a medium saucepan over medium heat, stirring until mixture is smooth and completely melted. Remove from heat and allow to cool.

In the well-chilled bowl of a stand mixer fitted with the whisk attachment, beat the heavy cream until stiff peaks form. Transfer whipped cream to another bowl and return bowl to mixer. Switch out the paddle attachment, and beat the cream cheese, powdered sugar, and hot chocolate mix together until light and creamy. Add half of the whipped cream, beating until just combined. Remove bowl from mixer and fold in the remaining whipped cream.

Add the egg whites* and sugar to a large heatproof bowl and set over a saucepan of barely simmering water. Heat mixture, stirring constantly, until the temperature hits 160ºF on a candy thermometer, about 3 - 5 minutes. Remove bowl from saucepan, add the cream of tartar, and beat mixture with an electric hand-mixer (or in a stand mixer) on high speed until stiff glossy peaks form, about 6 - 8 minutes. Add the vanilla near the very end.

Cut the cake into bite-sized cubes. Place half of the cake cubes into the bottom of a large trifle dish or bowl. Spoon half of the hot chocolate filling on top, spreading evenly. Drizzle half of the fudge sauce evenly over the filling. Spread the marshmallow fluff on top of the fudge sauce. Arrange the rest (or as much as you can fit) of the cake cubes on top of the marshmallow fluff, followed by the rest of the hot chocolate filling. Drizzle the rest of the fudge sauce on top.

Pipe whipped cream along the border of the trifle and top with chocolate sprinkles and mini marshmallows. Refrigerate until ready to serve.