Ok just got an order for tomorrow, 13 x 9 chocolate cake I only have one cake mix it is chocolate butter cake mix but how can I extand it or am I better off going back to the store?

Have no fear!
Add:
1 c flour
1/2 c sugar
1/2 c cocoa
an extra egg
and an extra 1/2 c milk.
If you want (this is what I do) you can add 4 oz of melted chocolate too.
bakes up real nice

I would use the 1c sourcream, 1/2c butter, 1 1/3 water, 3 eggs - beat together - then add 1c flour, 1c sugar, 1/4c cocoa powder, and bag of mix

This may even be a little too much for the 13x9

Good luck!

thank you for your quick replies! I had just come back from the store when my friend called in with the order! I didn't want to go back to Crest for more cake mixes!