So I tried to make one of those cakes that look like a mug of cocoa (lots of pics of them on CC). I used three 6" rounds layered (not torted) with a dowel down the center just to keep 'em centered. Fondant mug handle (inserted with lollipop sticks). Frosted in buttercream, smoothed, then decorated.

Everything looked fine and dandy, and I was quite proud of my amateur but cute creation. Then I picked up the cake round to transport it. As I was packing it in the box, I noticed the bottom has started to crack! There was a big crack in the buttercream around the circumference of the bottom of the mug.

Hubby says it was because my layer of frosting was too thick. Is he right? I think the swaying of the cake caused the cracks in the crusted buttercream, and that it might have been ok if I had either better supports or a stronger cake round.

Anyone know what might have gone wrong?

You had a "cake round" on the bottom? As in a cardboard cake circle? That's the problem. For any cake, but especialy one as heavy as what you're describing, you need a solid foundation - several cake circles glued//taped ogether and covered, a cake drum or covered masonite circle.

Ah... the mistakes of a newbie. It was, indeed, just one little 8" cardboard cake round. Thanks for the advice!

ETA: Just to clarify, so I should be using a solid cake board for all my cakes, even just regular 8" layer cakes? Or just for the heavy/awkward/tall ones?

At a minimum, I double up two cake circles but for the heavier ones I'll use either 3 or 4 cake circles or a drum, sometimes it's easier.

Ah - sounds like you definitely needed more support under the entire cake. I learned the hard way after doing a sheet cake and FINALLY getting that perfectly smooth buttercream, only to pick up the cake and have it start cracking. I had it on a cardboard cake board but should have had it on a covered sheet of MDF or plywood or something much more substantial.

However, in the future if you do have cracking buttercream (with proper support), we were told at the Tulsa cake show to put a little bit of vinegar in the buttercream to keep it from cracking. I would never have thought of that but I believe it was Marina Sousa who said that and she says you don't taste the vinegar.

Good luck and keep baking!

I use foamcore board (art supply department) so I don't have to tape cake circles together and I can make them anysize I want. If you watch Michaels ads they run them on sale all the time.

Yep, agree w/the others you did not have near enough support for such a heavy cake.
For my cakes that are 4" tall I mostly used one circle - except if it was over 12". So an 8x4; thru 10x4 went on one cake circle.
Anything else went on dbl boards glued together - well mostly I used dbl face tape - that's sticky on both sides.

did you dowel the three cakes, or just one dowel though to keep them secure? That could be a problem as well, even small cakes needs to be doweled when stacked 3 high. But I agree with the others, the flimsy board at the bottom was the biggest culprit IMO