I never use a heating core. I don't want the big hole(s) in my cake. I use greased, inverted metal flower nails. One or two in smaller pans (9" - 12") and more in larger pans. Spread out the nails to evenly spread the heat. The holes left by these nails is way smaller than those left by a heating core. The nail holes are easy to hide with buttercream and/or fondant.
11/11/09 at 5:04pm