Bake the Cake:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
2. Mix Dry Ingredients: In a large bowl, sift together flour, sugar, baking soda, salt, and cocoa powder.
3. Mix Wet Ingredients: In another bowl, whisk together vegetable oil, buttermilk, eggs, food coloring, vanilla extract, and vinegar until smooth.
4. Combine: Gradually add the wet mixture to the dry ingredients, mixing until just combined.
5. Pour and Bake: Divide the batter evenly between the prepared pans. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Let cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Make the Frosting:

6. Beat the Butter: In a large bowl, beat the softened butter until creamy.
7. Add Marshmallow Fluff: Mix in the marshmallow fluff until fully incorporated.
8. Add Sugar and Cream: Gradually add the powdered sugar and heavy cream, mixing until smooth and fluffy. Stir in vanilla extract.

Assemble the Cake:

9. Layer the Cakes: Once the cakes are completely cool, place one layer on a serving platter. Spread a generous amount of marshmallow frosting on top.
10. Add the Second Layer: Place the second cake layer on top and frost the top and sides of the entire cake with the remaining marshmallow frosting.

Create the Spiderweb Decoration:

11. Melt White Chocolate: In a microwave-safe bowl, melt the white chocolate chips in 30-second intervals until smooth. If desired, add a drop of black food coloring for a spooky twist.
12. Pipe the Web: Use a piping bag or a plastic zip-top bag with a small corner snipped off to pipe a spiral shape on top of the cake. Start from the center and work outward. Then, draw lines from the center to the edge to create a spiderweb effect.
13. Decorate: Add edible spiders or other decorations as desired.