Chicagoexpat,
I have never made sorbet/ice cream at home, but I used to be a pastry chef around town. Here's my recipe that I used while working at the now-defunct Soul Kitchen:
1 1/4 c. sugar
2 1/4 c. water
1 c. cocoa powder (the higher quality, the better.)
1/4 c. Vanilla Brandy* (I made my own by steeping "used" vanilla pods in a bottle of Brandy.) You can use any liquor you like - it helps to create a smooth texture and keeps those ice crystals down.
1/2 c. milk (Yes, I know there is no dairy in a true sorbet. Try this recipe - you'll understand.)
Bring the water and sugar to a boil, whisking. Combine the cocoa, brandy and milk. Add the sugar syrup to the cocoa mixture. Strain (preferably through a chinois.) Chill at least 4 hours, or overnight, before processing.
I broke the quantities down pretty far from my usual commercial batch, but you may wind up with more base than you can use at once. It will store in the refrigerator for several days - just give it a good whisking before running in your machine.
* If you make Creme Brulee at home, or any other dessert requiring the seeds from vanilla pods, do not throw out the pods! Toss them in a bottle of brandy or Vodka. Fresh berries tossed with a little sugar and some of this "vanillafied" alcohol are awesome! Or, of course, you can just drink it.