OK, I just ruined another batch of buttercream. I do not think I am letting the cornsyrup/sugar mixture get hot enough because the buttercream is very thin. What do you think? Also, with using egg yolk this frosting appears very yellow, does it stay that color or after it gets cold does it lighten up?

I'm no help... I have never even heard of Neoclassic buttercream.

Neoclassic buttercream recipe by Rose Levy Beranbaum:

http://www.starchefs.com/chefs/RBeranbaum/html/neoclassic_r_beranbaum.shtml

Discussion on neoclassic b/c on Rose's chatboard:

http://www.realbakingwithrose.com/index_ee.php/forums/viewthread/561/

Discussion on neoclassic b/c on egullet chatboard:

http://forums.egullet.org/index.php?showtopic=82087

HTH