I haven't done one myself, but I would roll out fondant to the thickness I wanted and use one of my piping tips to cut out the circles.
You can also write to some of the cakers in here who have already done Lego cakes and ask how they did it. The one on here yesterday was done really well.
You could possibly use Rice Krispys as well... Depending on the size you are making you could mold them in a cupcake pan, ice them along with the cake, and then cover all of it completely with fondant to get a smooth look.
About what cownsj said...you wouldn't have to make your mold, italready exists!
http://www.amazon.com/dp/B000TOHYGU/?tag=cakecentral-20
It is just a Lego ice cube tray, but I use it with candy melts all the time to make little chocolate treats.
But for a large style cake...I would either use rice crispy treats or find yourself a round cutter and make like mini cakes and cover those in fondant separately.
About what cownsj said...you wouldn't have to make your mold, italready exists!
http://www.amazon.com/dp/B000TOHYGU/?tag=cakecentral-20
It is just a Lego ice cube tray, but I use it with candy melts all the time to make little chocolate treats.
But for a large style cake...I would either use rice crispy treats or find yourself a round cutter and make like mini cakes and cover those in fondant separately.
Oh, how cool, that ice cube tray. That sure makes that part easier. Thanks for the info.
I've seen it done well using cookies, and using just fondant (rolled thick and cut into circles). I did one using the same cake as the base "brick" baked in a thin layer in a square pan and then cut out the circles using a biscuit cutter. (My son, whose birthday cake it was, did not want any fondant.)
You can also use marshmallows, cut in half and frosted -- if the proportions of your cake are right for that. Or a lot of people use cupcakes, but that is not ideal because of the slanted sides -- Lego studs have vertical sides.
If you use anything other than the fondant circles you'll have to frost it, and getting a smooth surface on those little circles can be a challenge. I did mine by putting each stud on a flower nail to frost, and then slid it into the freezer. The frozen studs could then be placed on the cake without messing up the surface.
I think it might work to use ganache or a poured fondant (like petit fours) as well, to cover the studs at the same time as the main brick part of the cake, but then the challenge is to get sharp corners. Rounded corners won't look quite like real Lego!
i'm doing a lego cake this week and I found lego candy at this awesome nut/candy store we have here. They are pretty close to real-sized little lego bricks in assorted colors and they even stack together 
Good luck with yours!
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