I am by no means an expert.  I have been baking for a long time, but cakes only the past few years.  I always make the Blackout Cake recipe from 365 Great Chocolate deserts cookbook and it usually is perfect!  however, lately, I am having a hard time.  Today, I made it and FAIL! I needed 2 -6" cake layers.  I looked up how much batter for that size pan, said 2 cups, however, it was way too full so I took a little out so it was about 2/3 full.  I used the wilton bake even strips.  The cakes were rising beautifully, I switched them around to make sure they both baked evenly.  Then the center of both fell while still in the oven! I think it was either that I moved them, or opened the oven too much.The other thing I did was tap the pan to get out bubbles, it says to do that in another recipe I have.    I baked the rest of the batter in a 10" round it it sank too.  However, I forgot my heat core for the 10" pan.  I made 2 changes to the recipe and wondering if that made a difference.  1 - I added King Arthur cake enhancer 2- it calls for boiling water but I added 1 T of dried espresso powder to the water.  I just don't  get all of the science of baking sometimes.  and wondered if the espresso could have messed with the leavening.  it calls for  baking soda.  anyway, I have to remake it for an order and wondered if I should try this recipe again, but without my changes, or try a new recipe.  Any tips or advice on what went wrong would be appreciated.  At least I can save the cake for cake pops!

i have a treasured chocolate cake recipe that will only bake successfully in certain size cake pans and only very low amounts of batter--pan filled under half full--

 i don't think the espresso powder would interfere if you reserve some of the water and add it to that separately from the cake enhancer--does the recipe call for the 'enhancer'?

you of course tapped the cake pans before you put them in the oven

Thanks everyone.  I guess I added the enhancer to make it even better.  I'll leave it out next time. 

my other question is this: I noticed that the cake reached the top of the side of the pan and shortly after it was when the center fell.  I had the same problem with some cupcakes I made the other day too.  They reached the top of the liner and then fell.  Too much batter maybe?

AI went true the same situation I think the heat isn't consistent enough.

When do you open the oven door for the first time?  Every time you open the oven, you can potentially change the temperature by 20 or more degrees...  and if you open the door too soon or close it too hard, the cake will fall.  Next time, fight temptation and do not open the door until the timer goes off.

Also, is your leavening still good?  It may be possible, too, that you over mixed, adding way too much air to the batter.  That would explain the batter reaching up the sides but collapsing in the middle.