For my birthday, the kids made a dark chocolate fudge cake and my favorite chocolate glaze for the frosting...
Ghiradelli's Cocoa powder, a little Hershey's dark chocolate cocoa for the dark dark chocolate flavor, a couple tablespoons of butter, fat-free half and half (it's what I had. If you have heavy cream or regular half and half, use that. Milk works too.). Baker's sugar or powdered sugar, and a dash of vanilla.
Melt butter (or margarine) over low heat, add the other stuff and beat well. For a glaze, add a little more milk/cream. For frosting, add a little more sugar and cocoa. I never ever measure when I make this, I just throw stuff into the pan. I taste it, of course, to make sure it's not too sweet or not sweet enough.
I cut the cake and put pieces on a platter, then drizzle it liberally with the glaze. It looks kinda messy, tastes really fabulous.