½ cup cocoa powder
1 cup water
2 cups sugar
⅛ teaspoon salt
¼ teaspoon vanilla
Mix the cocoa and the water in a saucepan. Heat and stir to dissolve the cocoa. Add the sugar, and stir to dissolve. Boil for 3 minutes over medium heat. Add the salt and the vanilla. Let cool. Pour into a sterile glass jar, and store in the refrigerator. Keeps for several months. Yields two cups.
The result is very rich, so I only use a teaspoon for a glass of chocolate milk.
source (not a vegan site): http://smallnotebook.org/2009/07/10/homemade-chocolate-syrup/