I saw lots of them in the galleries, but most look like fondant. Is there a simpler way to make a Lego cake? My customer wants a 1/4 sheet and doesn't want fondant--is there any way to make one in buttercream that looks decent? I am trying to picture smoothing the buttercream around each peg... 
I made a lego block using a two layer 9x13 and 6 cupcakes, all iced with buttercream smoothed using the Viva method. My client and the birthday boy loved it.
Have a look:
http://www.sugarfrosted.com/displayimage.php?pos=-2627
Click on the picture to enlarge it to see details. It was a very warm day so it looks a little shiny, but then again, legos are shiny! 
I did a lego cake where the decorations were fondant, but the cake was iced in buttercream. Like you, I didn't want an entirely fondant covered cake, but I wanted smooth lego pieces.
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1745602
Fantastic! These are exactly what I was looking for--thank you.
Did you charge more than your usual per serving charge with the cake that had the cupcakes on top?
I did a Lego cake for my son with no fondant, per his request. I didn't know how to do the Viva method at the time so the frosting wasn't so smooth, but he was happy. 
Anyway, to make the studs I baked a thin layer of cake in a square pan and then cut circles out of it using a little biscuit cutter. To frost them, I stuck one at a time on a flower nail, and it wasn't too hard to get them smoothed that way. Then slid them onto a board and froze them. The frozen studs could be handled and placed on the cake without making too much of a mess of the frosting, and then they could be smoothed a bit more in place after they thawed.
Cupcakes work, but you need to trim the sides if you want the dimensions to be authentic, because Lego studs don't taper. You can also use halved marshmallows, or round cookies. Or, of course, you could use fondant, rolled thick and cut into circles, but if the cake itself is frosted with BC that isn't going to look quite right -- the color/shine difference is too obvious.
Fantastic! These are exactly what I was looking for--thank you.
Did you charge more than your usual per serving charge with the cake that had the cupcakes on top?
This cake was no big deal to me, so I charged my normal per-serving fee. However, my client gave me a very BIG tip, because he is one person who really appreciates what I do...and he says I don't ever charge enough. Of course, he is from Chicago and probably would have paid a whole lot more there.
I trimmed the cupcakes slightly so they'd have straighter sides, set them upside down onto a cookie sheet(no sides) and used a generous layer of stiff-ish buttercream. Smoothed and shaped each cuppie with Viva. Then chilled the iced cuppies a bit in the fridge to make handling easier. I had already iced & smoothed the 2 layer 9x13. I added a little dollop of buttercream to the spots I wanted to set the cuppies. Then I slid a small angled spatula under each cuppie and gently slid them onto the dollops. I used the Viva (as little as possible) to handle/move the cuppies to position them properly. Then I cleaned up any marks or blemishes I'd made in the placement. Then I added the few details. and wrote on the board.
Piece of cake!
what is the Viva method?
http://cakecentral.com/articles/71/how-to-create-faux-fondant-the-paper-smoothing-towel-method-viva
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