Well, here is directions to make the heads.
http://www.livinglocurto.com/2010/06/lego-cake-pops/
7yyrt, Those marshmallow lego heads are so awesome!!! Thank you for sharing!
yvonne123, there are of course the numerous lego brick cakes you can find in the cake central galleries. But if it's HP themed, I'd pick a HP minifig or one of the Lego HP sets for inspiratoin. http://harrypotter.lego.com/en-US/default.aspx
But I like this photo of the three main characters as lego-minifigs. You could make it more 3D or as a detailed sheet cake...
http://cache.lego.com/upload/contentTemplating/VideoGamesGames/images/pic555A287743C9F347539F9F4142270970.jpg
Good luck! Sounds like a super-fun cake!
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