Hi everyone!
I have noticed a problem with sinking cakes, but it only happens when I bake chocolate cakes. Last week, I had to bake 2 round 8in chocolate layers and then 1 12in square chocolate layer and 1 square 12in french vanilla layer. The french vanilla layer came out perfectly (used flower nail trick)! But all of my chocolate layers sank after they came out of the oven! They were done...I checked that.
I've tried to bake them a little longer, but the edges come out hard and crunchy and the tops tend to split. I've also tried to use the baking strips around my pans too.
They were DH box mixes with a box of pudding and an extra egg added. I baked them one at a time (8in cakes were done together) at 350. And a flower nail was used in each of the 12in cakes.
Thanks so much for any suggestions!
I tried a Betty Crocker mix once and was so unhappy with it...maybe it was a fluke! I should probably try it again...
I followed the ingredients exactly as it states on the box, however I added one small box of instant pudding and an extra egg.
I was thrilled with the texture and flavor, but the center just sank...lol
" I followed the ingredients exactly as it states on the box, however I added one small box of instant pudding and an extra egg. "
Then I would suggest not doing that ! 
Just teasing you scarlett..... I've had it happen to me from time to time to, and never found out why..... Sure makes me mad though !!
Sorry about your cakes. I have come to the conclussion about myself that I can not bake chocolate cakes to save my life. I tried another one a week ago and this time it took forever to bake and still had the burning smell (before it was remotely done) as it was cooking. WHY GOD WHY 
I use your same recipe with the DH mixes. The only difference I do besides what you have stated is I always bake my cakes at 325. I know it takes a little bit longer but i seem to have more consistant results with it.
I also noticed that after I changed brands of cake pans it has a lot to do with your pans also. I was using wilton pans and had the problem sometimes. Since switching gradually over to Magic Line I haven't had the problem. I only seem to have the problem with wilton cake pans for somereason. It must be the way that they distribute the heat.
Something else you may want to try is a couple more flower nails near the middle of the cake. I tend to over do the flower nails i'm sure but I am one that would rather be safe than sorry.
Hope this helps some.
Chocolate cakes are fussy--for me anyway! I use ML pans, a flower nail, magic strips, 325 oven, and I don't fill the pans more than 1/2 full. I do add pudding and an extra egg, along with one T of meringue powder. I never open the oven door until the cake smells done + 5 minutes. I cook in round pans only; I haven't tried these strategies with square pans.
I read, a long time ago, that some people will place a small pan of hot water in the lower rack to help with cracking.
I baked 2 8in square german chocolate cakes last weekend and had that happen to me...the only thing I had done that would have affected them differently than normal is allowing my dd to open the oven and look at them. I do believe that the loss of heat is what caused them to sink. The second set I did...the same day came out perfectly but I didn't open the oven AT ALL.
Thank you so much for the tips!
I don't believe that DH has pudding in the mix...
I may try the magic line pans! I have the big 12x18 sheet pan by ML and it seems to work ok. It's been awhile since I've made a sheet cake though so I don't remember if my chocolate cake sank or not! 
Just think of the sunken part as an extra place to put filling.
That happened with the two I made for my bosses birthday!! He was quite curious as to how I got so much extra filling in that one spot...
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