Make your chocolate cake (2 layers, whichever is your go-to recipe or a box cake) and allow it to cool completely.
Frost the top of one layer with vanilla frosting and then top it with the other layer.
Frost the whole cake with chocolate frosting.
Mound popped popcorn on top of the cake OR stud the whole cake with it. Any flavor works, plain, butter, kettle...
Choose to add extras like a caramel drizzle or a chocolate drizzle or both.