more moist cake?
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I may be criticized by some fellow bakers for this. I came from a kitchen where butter ruled all, and the word "shortening" was only whispered. But what I think you might need is to use a High-Ratio Shortening, meaning that it can hold more liquid and it helps extend the shelf life of your cakes. There's 2 types I know of, Sweetex/Vreamay/Alpine is a replacement for a plastic shortening--Like your common crisco. I'm not sure the formula for replacing butter/shortening with the Sweetex/Vreamay/Alpine off the top of my head, but I can post it if you really want to know.
The other type of shortening Nutex/Fluid Flex which would be a trans-fat replacement for a liquid shortening-such as oil.
Both of these shortenings are High-Ratio which means you can add just a tiny bit more liquid to them, and these cakes made with these products will have excellent keeping quality and the cake freezes well.
Your local bakery should carry these products. And I am sure they would happily sell some to you.