Please help. I am making my sisters wedding cake which is quite complicated and I need to have it finished and hidden before they arrive which is 4 days before the wedding. I really need a chocolate cake filling that is not terribly too sweet and will hold up to being frozen while i decorate and store it? Also the wedding is in 10 days so I dont have a lot of time to experiment !! 
Buttercream freezes well.
I've also frozen the chocolate chip cookie dough, which is yummy and turned out well.
http://cakecentral.com/recipes/7600/chocolate-chip-cookie-dough-icingfilling
In that same cake a had a fudge filling, but it seemed to absorb into the cake a good bit once it was thawed.
http://cakecentral.com/recipes/15805/awsome-dark-chocolate-filling
I haven't tried this one frozen, but might soon. I think it will hold up well
http://cakecentral.com/recipes/5773/oreo-filling
Also, you can stir fruit jam or spread into your buttercream if you want a fruit flavored filling. Those should freeze well, also so it's base of buttercream.
HTH. Interested to see what other answers are given. I usually freeze my cakes stacked, but it's usually just buttercream. Will be good to have some more ideas!
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