So, I'm at a birthday party for a classmate from my son's Pre-School.... Standard sheet cake was being cut for the kids and they have lots to spare so they hand some out to the parents.
Chocolate cake with some type of creamy chocolatey filling. As I'm sitting there chatting with the parents, I take a taste of the cake. And you know what???? The cake was good, but the filling....OMG, the FILLING WAS AMAZING!!!!!!!!!!!!!!!!!!!!!!!!!!!
I groaned out load. 
One of the Dads looked at me and nodded. "It's really good" he agreed.
YES YES YES!!!!!!!!! VERY GOOD!!!!!!!!!!
I don't know what it was, but it was creamy and thick and it just hit the spot! What was it????? How do I make it???? It was almost like a thick, whipped cream kind of thing, but more stable. I don't know what I'm talking about, but I'm temped to go to Costco and just order a cake made entirely out of that filling.
I need help. 
Who knows what I'm talking about and knows how to make it. I must have some immediately!!!!!!!!! 
angie
I don't know for sure what it is, but it could be Rich's Chocolate Bettercreme or Chocolate Pastry Pride.
Good news is that if you really like it, I'm pretty sure you can go to the Costco bakery and purchase a bucket of it--they'll usually be able to sell you anything they use in their products.
Keep us posted.
Rae
Thanks, Rae. I've never worked with either Bettercream or Pastry Pride. I just really enjoyed it because it wasn't sweet at all.
I won't launch into any more descriptives. LOL. I'm drooling!!!!
Hi, Angela.
Here's info from Rich's website:
http://www.richs.com/foodservice_products_1.asp
Bettercreme Pre-Whipped
pre-whipped, so it's ready to use
available in Vanilla, Chocolate and Double Rich Chocolate* in 15 lb. & 9 lb. pails that are simple to open and reseal
* Double Rich Chocolate only available in 15 lb. pails.
I can get the small amounts at GFS (Gordon Food Service) in Mentor, but they don't carry the chocolate or the large quantities.
Rae
I am SOOOO in love with the chocolate mouse filling that they use at BJ's Wholesale. 
When I told my wilton instructor you would have thought I just told her I killed her dog.
I can't help it! And I'm not the only one, I know there are other bakers out there with the closet obssesion for BJs filling! 
Costco has 3 types of fillings for their cakes:
strawberry mousse
apricot mousse
chocolate mousse
That's not to say that the cake wasn't ordered w/ one of their chocolate frostings (chocolate BC, or chocolate whipped) But, you probably were tasting the mousse, from what you described.
well I bought a big can of chocolate pudding from the shelf at sam's club for filling the cakes and it was amazing, sounds pretty much to what you describe like a whipped mousse, I dont know but it was delicious...
Thanks for the replies everyone!!! It was kind of moussey...moussy..... 
It tasted like a mousse. It was so thick, too. I couldn't quite understand how there wasn't any bulging on the side with all that filling.
I will have to make a trip soon. Luckily I'm about 5 minutes away, so it will be well worth it.
Thank you again, everyone!!
Angie
I think it's whipping cream, a box of instant pudding and something else. I can't remember but there was a post about strawberry filling and someone answered that's what it was. They worked at Costco I think. Maybe you could search the forums for Costco and see if you can find it. It was a while ago, but I'm sure it will still be there.
I think it's Rich's chocolate Bettercreame. I happened to make a cake and used the this Bettercreame filling and I think it tasted exactly like the Costco cake filling. It is a non-dairy whipped topping.
I love using it as a filling. Oh - and I love the Costco cakes too. I know, I know, don't shoot me for saying it. But they do taste pretty darn good.
You can get Rich's bettercream at Sysco. Call them as each state operates independently and has different Cash & Carry rules and minimums.
In MD. & MN. you can get it in small qty, while in Utah you have to buy the whole case. Just call ahead to see if it is worth your while.
Hope this helps.
Claire
I don't know what Costco's filling is like but a really yummy chocolate filling is ganache! It's 1/2 semi-sweet chocolate and 1/2 heavy cream. You just melt the chocolate into the cream until it's smooth then let it cool. Then when it's cold, you can even whip it up like whipped cream if you want to. It's the ultimate death by chocolate yum!
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