1. Prepare the devil’s food cake mix according to box instructions using eggs, water, and oil. Divide the batter between two greased 9-inch round cake pans and bake at 350°F for 25–30 minutes. Let cool completely.

2. In a saucepan over medium heat, melt butter with sugar and milk. Stir constantly until sugar is dissolved. Whisk in flour and cook for another minute until slightly thickened. Let cool to room temperature.

3. Transfer cooled mixture to a mixing bowl. Add marshmallow fluff and vanilla extract. Beat until the filling is light, fluffy, and smooth.

4. Place one cooled cake layer on a serving plate. Spread the filling evenly across the top. Gently place the second cake layer over the filling.

5. In a small saucepan, heat the heavy whipping cream until just simmering. Pour over chocolate chips in a heatproof bowl and let sit for 1 minute. Stir until smooth to make the ganache.

6. Pour ganache over the assembled cake, letting it drip down the sides for a smooth, glossy finish. Refrigerate for at least 1 hour before slicing and serving.