Bake the Base: Preheat your oven to 350°F. Grease two 12×17-inch rimmed sheet pans and line them with parchment paper, allowing a slight overhang for easy removal. Prepare the cake mix according to the box instructions, making sure the batter is smooth and well combined. Divide the batter evenly between the prepared pans and spread it into an even layer. Bake for about 10 minutes, or until the cake is set and lightly golden on top. Avoid overbaking, as the cake should remain soft and flexible. Remove from the oven and let the cakes cool completely.
Whip the Filling: In a large mixing bowl, beat the heavy whipping cream, powdered sugar, and vanilla extract on medium-high speed until soft peaks form. The cream should be light and fluffy but still smooth. Add the marshmallow fluff and continue beating until stiff peaks form. The filling should be thick, stable, and creamy, perfect for spreading without sliding.
Assemble: Place one cooled cake sheet on a flat surface, keeping the parchment underneath. Spread the filling evenly over the entire surface in a thin, uniform layer. Carefully place the second cake sheet on top, aligning the edges as closely as possible. Gently peel away the parchment paper from the top layer, taking care not to tear the cake.
