For the Box all you need is a round cake and for the popcorn part I use mini marshmallow I cut one end in 4 criss cross shape with a scissor and pull apart a little and color the inside of the popcorn with a little yellow food color for the look of butter. I also add clear sprinkles on top to look like salt. before putting the popcorn on the cake I fill the top with yellow butter cream to make it look more full of popcorn.
I hope this helps you out on how to make the popcorn for the cake.
for the Hollywood theme their are great ideas in the cake gallery that you can look on how you'll like it to look like.
I'm going to try to add a photo of a cake that I have done.
I have used both marshmellows and fondant for the popcorn. They are both simple and easy. For the fondant idea I use three balls one slightly bigger than the other two. I connect the two smaller balls to the larger balls to almost look like mickey mouse ears. Then use the ball tool and make indents in the two smaller balls to make popcorn. I used brown paint and painted the indents to look like kernals. I then airbrushed the popcorn (or you can paint it) yellow. The box the idea from above looks great, I have only used rice krispy treats because my popcorn bucket was only part of the cake.