Do I need to rebake or can I just level it? Or even make up the difference with buttercream filling?

Thank you!

Just wondering what size the cake was? Is it cooked all the way???

What size pan did you use? Did the cake sink before or after it came out of the oven?

If the cake sinks after it comes out of the even it could be underbaked. You can check this by torting (cutting your cake in half).

There is one cake mix that I have trouble with in the 8x3 pan, that one sinks every time, but usually before it comes out of the oven and that is DH's Golden Butter Recipe.

Its an 8" round. I cooked 2 and one was a little less filled and it came out fine. I cooked this one a little longer. It seemed done.

My first crack at chocolate WASC sank too, so I just leveled it off and it was fine.