Today I was making a test cake, to see how it turned out, for possible use on a birthday cake. Everything went wrong. I'll attach pictures. The recipes I used from CC were:

White Velvet Cake
Italian Swish Buttercream
Stawberry Mouse filling
MMF

First the cake. It seemed to be a very thick batter. It did not even pour into the pan. Once I put it in the ton the oven 2 min later I realized I had forgot to add the sour cream. So I pulled it out, and back to the mixer for the Sour Cream. It then looked more typical of cake batter, but still slightly thicker. The recipe said it was a dense cake so I figured it was OK. Well here are the problems. It did not rise with a bump, but instead a depression. It also did not look white. Even though I used clear vanilla.
It was so dense the cake slicer would not slice it. I had to use a bread type knife to cut it in half for my filling. At which point one of the layers crack in 3 places. SO I set that one on the bottom, and my filling and on to the BC

The ISBC, looked fine. I put in the fridge since I was not going to ice for a while. Big mistake. The consitency changed even though I tried to re whip. It kept sliding off the cake. I put the whole thing in the fridge while I worked on the MMF.

This was the easy part right. Well my marshmellows never seemed to quite liquify. The more I nuked it the more they puffed up. More like the stuff in the jar. After whipping and kneading it seem to survive. And I have to say it turned out OK. I liked working with it, and it seemed easier that regular fondant.

So my Q are why is the cake SO dense and not white, and what was wrong with ISBC. Does it not work under MMF? My son the taste tester said, the cake tasted OK, and he did not taste the white chocolate at all.
Sorry the photo did not attach. Let me work on that

Can you post links to the recipes?
Are you in China? how old is the baking powder?

http://cakecentral.com/recipes/15190/white-velvet-cake
No baking powder involved. The only thing I did different was use meringue powder with the required water for the equivilent of egg white.

http://cakecentral.com/recipes/5453/the-well-dressed-cake-swiss-meringue-buttercream-with-variations

http://cakecentral.com/recipes/1949/marshmallow-fondant-mmf

http://cakecentral.com/recipes/7308/costco-mousse-recipe
here I added 3 Tblsp on strawberry gelatin (powder) for strawberry flavor.

I'm having trouble adding the picture, it won't attach so I'll keep trying.

And I'm not in china, it's just a code name I used for my blog while I was adopting our daughter from there

Awww, you just got my hopes up, I was thinking, yeah, finally a cake buddy near me

Perhaps it was the meringue powder instead of real eggwhite and taking the cake out of the oven to remix is never a good idea. Perhaps the bakemix is off, the baking powder would be in that. I don't think you would be able to really taste the white choc though, it's a real subtle flavour as it is. The original poster did say, it would be a hint of white choc flavour

ugh I wish I could get the picture up

Oh your in China? We were there last year. I would love to learn how to make the gorgeus cakes they have over there.
Oh I think I got the pictures up.

Try again, and make sure you get all the ingredients this time, and make sure your baking powder is fresh - you may come out with a wonderful cake! It may have just been the combination of these things that did it. Whenever something doesn't turn out right, I usually try to go back and redo it making sure to check anything that could be causing the problem.

Yeah they can decorate, but the stuff they use is Blaeh

It's all chemical I think, comes in a literpack and whips up to about 3-4 x the volume. Gives us indigestion, but they sure can pipe beautiful things with it

what is italian swiss buttercream?? If it is anything like swiss merigue buttercream the problem was when you put it in the fridge.... once the temp goes down on it and you dont let it get to room temp again it will start to seperate... and get wtery and such.... if that ever happens again whip whip whip... you can also nuke for 10 sec intervels.... hth

Yes it was. I did try to whip it, but it started loosing it's smooth texture, and I was worried it was going to looking like cottage cheese. It was the first time I made this type of meringue, so now I know. Thanks for the tip

btw, your fondant looks great!

Yeah they can decorate, but the stuff they use is Blaeh

It's all chemical I think, comes in a literpack and whips up to about 3-4 x the volume. Gives us indigestion, but they sure can pipe beautiful things with it

I know what you mean; Asian cakes always look stunning and then taste like cardboard topped with grease. I do enjoy Japanese cheesecakes, which generally aren't decorative at all.

As for the OP's post: It sounds like your cake fell, not surprising given the late addition of the sour cream, which is essential to your cake rising. Try the recipe again, and if you still have problems use actual egg whites and not the meringue powder.

I don't know anything about ISBC. Usually buttercream requires whack-loads of whipping.

It sounds like your MMF turned out great! When I make it the marshmellows don't really liquify, they "puff" and only become more liquid when I stir them.