Bottled yesterday -- let's hope this turns out:

"The Sludge"
-Rainbow Brand Cream Soda Extract (2 fl oz, 4 gal recipe)
-Boil 3 cups water with 3 heaping tablespoons unsweetened cocoa powder, 1 lb. lactose sugar, and 2 cinnamon sticks for about 20 minutes, then bring water to 4 gallon line and add the white sugar.

The cocoa powder appears to easily separate from the cream soda. Looks like you'll need to lightly agitate the bottle before consumption, and drink with a stir-stick to keep the brew as homogeneous as possible. Will post back in a few days when the soda is carbonated.