Definitely use a heat core for that size cake!
I tried the flower nail once, and it got all tangled up with my cooling rack somehow and ripped a big hole in the cake. After that, I emptied out a salsa can, cut the bottom out of it, and used that for a heat core. You just grease and flour it like the pan and stick it down in the batter once it's poured. The core is later lifted out of the baked cake prior to turning it out of the pan. It makes a very small "wound" in the cake that ices over flawlessly. I use the can in every single cake I make now (even the 8-inch) because it makes for a domeless cake in addition to ensuring a well-cooked middle.
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