Here's the filling I used for the Graduation Cap Cake. I wanted a strawberry filling, but I also wanted something light. So I came up with this idea, and it worked even better than I had hoped! (Don't you love it when that happens?)
1 can of Solo brand Strawberry Cake & Pastry Filling (you can find it in the baking aisle of your grocery store)
1 tub of Cool Whip
Move the Cool Whip from the freezer to the fridge one day before you want to use it.
On the day you make the cake, gently mix the Cool Whip with the strawberry filling. The result is a creamy, fluffy filling that tastes sort of like a mousse without all the fancy prep work.
Unless you are making a very large cake, you will not need all of the Cool Whip and filling. For the Graduation Cap Cake (which had a total of four layers), I used about 2/3 of the Cool Whip tub + 2/3 of the can of filling.
To fill the layers, place your first layer of cake on your cake plate or cake board. Put some buttercream icing in a decorating bag and pipe a nice tube of icing around the edge of your layer. (This will act like a dam to hold your filling in.) Then put your strawberry mousse filling in another decorating bag and fill in the area. If necessary, you can spread the filling with a knife to even it out. Place your next layer on top. If there are more layers, repeat the process.
I had some filling left over, so I stored it in a plastic tub in the fridge. It lasted for a while . . . I can't say exactly how long it would last because I ate it all in just a few days. :-)
Solo makes a ton of different flavors for their cake and pastry filling, so you could mix up all sorts of "mousses" with these. In fact, I'll be making a raspberry mousse for a cake this weekend.
Here's what the Graduation Cap Cake looked like after we cut into it. I'm pretty sure one of the guys at the party polished off all of what you see in this picture.