I would probably make a couple sheetcakes, cut them in half, frost tops, stack, and freeze them. Once frozen carve the sides down to make the pyramid shape. Someone else might have a better way, but I'm all about sculpting from frozen cakes. I've had a great results with this method, although have never made a pyramid. 
Here is something. I hope this helps. Go to www.texasmonthly.com/food and when you get there search for Baileys Pyramid cake. A little on the scientific side search for rule 90 cake. You will see the site listed there.
I would probably make a couple sheetcakes, cut them in half, frost tops, stack, and freeze them. Once frozen carve the sides down to make the pyramid shape. Someone else might have a better way, but I'm all about sculpting from frozen cakes. I've had a great results with this method, although have never made a pyramid.
This is how I constructed mine and I totally agree with carving frozen layers! There's a pic of it in my gallery although, not a very good quality picture but you'll get the idea.
I hope it works out for you! 
elly
Ellyrea, your cake are beautiful !! congratulation !
How did you make the effect of color i fondant on you butterfly cake?? it's beautiful.
Thanks
Doudoun
Thank you Doudoun. It's quite simple really, I just place several "dashes", rather than "dots" of color on a mound of fondant with a toothpick. Then I fold the fondant in around and over the color. Then I just pull to stretch out and twist a few times so is not to work the color in too much. Then roll out. It would look fantastic on a pyramid cake in a tan color!!!! 
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