In a saucepan, thoroughly combine the cornstarch, sugar, and cocoa.
Add the milk and stir until very smooth. Cook on medium heat, stirring constantly, until the pudding comes to a boil.
Lower heat and gently simmer, stirring continuously, for 3 or 4 minutes.
Stir in the vanilla, pour the hot pudding into a decorative serving bowl or individual custard cups, and serve warm or chill for about 2 hours, until cold and set.
Note: you can replace the skim milk with whole milk for a richer pudding. If you want a more intense chocolate hit, replace the cocoa powder with 2 or 3 ounces of high-quality semisweet chocolate, such as Scharffen Berger. Melt the chocolate separately, then whisk it into the other ingredients. Cook, stirring, until the mixture simmers and thickens. Add vanilla and serve.
There are 123 calories in 1 serving of Dark Chocolate Pudding.