Well you got three questions here and I can't seem to answer them because:
Anyway, how many days do cakes stay fresh?
It depends on the type of ingredients you use. For instance, my fondant cakes seem to last a lot longer then buttercream.
What's the best way to store them?
Cut or uncut? Uncut fondant cakes can be kept in a nice bakers box or cake saver for days....cut cakes on the otherhand need more protection. I have been making fruit cakes like the Brits and they last forever.
How long does Buttercream Dream last?
Can't help you here at all, I never have used the product. I make Sylvia Weinstocks buttercream.
Sorry I could not be more help.... 
Mix cakes will last longer than scratch cakes because of the preservatives in them.
Once a cake is iced, the icing seals the cake. Think of the icing as the same as the plastic wrap around your loaf of bread that sits in your pantry for days and days.
Made my daughters' baby shower cake on THursday for a shower on Sat; put a couple of pieces of leftover cake in the frig, saran wrapped. Ate it on the next wednesday .... tasted even better than day one.
Sorry, I meant to say un-iced cakes!
My daughter's birthday party is on Saturday, and I wanted to get as much done today as possible.
Buttercream Dream is a recipe I found on this site--had high ratings so I thought I'd give it a try!
Indydebi--Thanks! I always wondered that (icing sealing the cake)!
Anyway, how many days do cakes stay fresh? What's the best way to store them?
I always make my cakes ahead of time and then freeze. I have a wedding shower cake to make for Saturday, but I made my cakes yesterday and today. I also made my icing today to keep for Saturday when the cakes will be iced. As for storing them, I just keep an iced cake on the counter (never in the fridge) with a little bit of saran wrap over the part of the cake that has been cut into.
How long does Buttercream Dream last?
Can't help you out here either, as I use Wiltons recipe. I use water in my recipe not milk or any other 'perishable' item so my BC will last for a while just sitting out on the counter...I think it's at least a week or so.
HTH alittle
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