The classic topping for a petit four is a white sugar glaze. Fondant is what it’s called. What is fondant? Short answer: sugar. However it’s sugar that’s been manipulated in such a way that it’s soft and pliable instead of hard and crunchy. Fondant comes in two basic types: your rolled fondant and your poured fondant. Rolled fondant is basically a firm sugar putty that’s especially useful in the cake-making world, where it’s colored, rolled thin and applied to cakes in lieu of icing. It’s also used widely in candy making, especially the white centers of coconut creams.
Poured fondant is pretty much the same thing, save to say it’s got more water bound up in it. Though it’s not as versatile as firmer fondants (since it goes on thin and becomes almost brittle as it dries) it is the perfect glaze for a mini-cake like a petit four. There’s a good recipe here.
Oh, and if you’re wondering how fondant is pronounced, I can’t really help you. Lots of people on this side of the big drink like to use the French-inflected pronunciation, fon-DAHNT. I however use the Americanized one: FON-dunt. It’s up to you.
And of course I said I was going to put a non-traditional chocolate glaze on a few of them too. For that I’m not going to obsess too much, and simply go for a dark chocolate ganache. And that of course is another French word for a simple 2-1 (by weight) combination of melted chocolate and heavy cream. Works for me.