I love Costco cakes...and their fillings are soooo yummy. I found this on another site and doctored it up a bit. It can be any flavor you want or need!
Combine all in a bowl and mix with hand blender until smooth and thick.
For Vanilla - Use Vanilla flavored pudding.
For Milk Chocolate - Use chocolate pudding.
For dark chocolate - Use chocolate pudding and add 2 T cocoa powder (or more to your liking).
For cheesecake flavored - Use cheescake pudding.
Be sure to use it on cooled layer cakes and refrigerate it after it is mixed and/or on the cake. You can use fat free milk if you want to but it is no where near as rich, thick and creamy.