This frosts two layers or a 13/9. If frosting a 3-layer cake, than double the recipe and eat the extra!
1 cup sugar
3 tablespoons cornstarch
2 ounces unsweetened chocolate, grated (this is important! grate the chocolate on a microplane or on the fine side of the box grater)
1/4 teaspoon salt
1 cup boiling water
3 tablespoons butter
1 teaspoon vanilla
On the stovetop in a heavy saucepan: Mix sugar and cornstarch; add chocolate and salt. Add boiling water and cook until mixture thickens. Remove from heat. Add butter and vanilla. Spread on the cake while the frosting is still hot for a glossy frosting which remains soft and smooth.
I also haven't run into another chocolate icing recipe like this, but I'm sure versions must exist. If you use a recipe like this, let me know where you got it!
This makes a dark (nearly black) cooked-frosting-textured icing that is nothing at all like a buttercream. It's delicious, but don't try to do any kind of decorating with it. I've found this to be slightly more popular with "grownups" than with kids, as small children may find this a touch too strong-flavored for them if they aren't used to a bittersweet chocolate flavor.
I used this on the "Perfect All American Chocolate Butter Cake" from the Cake Bible. That was my $312 United Way raffle cake.