I have made WASC over and over again, and love it. Never had a problem before. I've used it with white, yellow, chocolate, etc... just editing the extracts.
I have 2 large cakes to do this weekend, one yellow, and one white. I made the white cake which came out fine. I made the yellow last night, and it sunk in the middle. When I cut into it, it was EXTREMELY dense. Much more than normal. It had almost a rubbery consistency. I made another batch today to replace last nights mess....and it was the same way! sunk in the middle and rubbery, dense, cake.
Has anyone had this happen before?! I'm so frustrated! I am going to make batch of my scratch recipe b/c I can't afford another problem, but I really love WASC when it comes out right and would like to try and understand what happened TWICE?!!!
Rubbery, dense and sunken are usually the results of overmixing. 
Handy cake troubleshooting charts:
http://tinyurl.com/2p5bdu
http://tinyurl.com/32goqe
http://tinyurl.com/6c745g
http://tinyurl.com/6lpjww
Additionally, incorrectly measuring the flour can also contribute to a "heavy" cake.
How to measure flour correctly:
When measuring flour, do you use the "scoop and drag" method and then shake to level.... You should be aerating the flour prior to gently spooning it into the measuring cup and using a straight edge to level.
When I make any of the WASC cake recipes, I sift all the dry ingredients together into a large bowl, and mix all the wet ingredients in a second larger bowl.
Then I add the dry to the wet and beat for 2 mins. using an electric hand mixer at medium speed.
If using a stand mixer, I would mix at the lowest speed for 2 mins. or less.
When it comes to mixing, MORE (as in more speed or longer mixing time) is not BETTER. Overmixing will develop the gluten and result in a tough cake.
HTH
Thanks for the info! That is probably it...I always set a timer when mixing the cake, and today I was rushing around and didn't set one. I probably let it go too long.
Just popped the new batch of cakes into the oven and keeping my fingers crossed it comes out right.
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