I was just reading a post about having clean edges when covering with fondant and noticed someone saying about Spackle - i'm wondering what this is? I usually just use a buttercream with a 50/50 butter/shortening mix.

Spackle was created by Toba Garrett in an effort to achieve the smooth, flawless finishes that European cakers get because they use firmer cakes that are also covered in marzipan. She has a recipe in both of her books, but basically its a paste made from your buttercream (it doesn't matter what type you use), a bit of whatever you used for the filling and a liquid. The liquid can be water, milk, liquor, whatever. The main ingredient is cake crumbs. If you trim your cake sides or have to level your cakes, those are the crumbs you use. If I have a lot of trimmings I run them through the food processor and save whatever I don't use. The paste shoudl be the consistency of oatmeal. If it is too loose, it's useless. Too thick and it will tear your cake. Toba puts a very thin crumbcoat on her cake prior to spackling. I don't.

Spackling occurs after filling, and allowing the filling to rest. When I remove the cake from the fridge, I trim the sides, throwing the trimmings in a bowl. I make the spackle, spackle the cake and put it back in the fridge while I prepare the final frosting. Spackle will fill cracks, and holes and even things out. I spackle all of my cakes that have any filling. It makes for a beautiful cake. People can never figure out what that extra 'filling' is. I've even used spackle as a filling!! HTH

Ditto DDiva. Spackling also reduces the amount of sugar you need to add to your BC to stiffen it.