A cake filling is used to add an element of flavor. When we say filling a cake what we are doing is
putting cake, then flavor, then cake flavor etc and so on until we place the top layer of cake and cover the whole thing in frosting. Fillings can be icing (the same or differently flavored from the outside icing), jams, fresh fruit, custards, curds (lemon, orange etc.) ganaches, mousses, puddings. Fillings are basically limited only by your imagination. Of course which filling you choose will determine how you need to store the cake -- in the fridge or on the countertop. Fillings should be of a fairly thick but spreadable consistancy so that they hold thier form under the weight of the cake.
I hope this helps.
Here is a great tutorial done by Cakeboss that goes through filling a cake=) HTH some, Jen
http://www.cakeboss.com/PreventBulging.aspx
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