COMBINE 1 cup of the sugar, cocoa, cornstarch and salt in medium saucepan. WHISK in combined milk and egg yolks. COOK over medium-low heat until mixture thickens, whisking constantly. BOIL 1 minute. REMOVE from heat. STIR in vanilla.
Tarts are best served warm, as refrigerating causes the shells to lose crispness.
Filling can also be spooned into small oven-proof ramekins, topped with meringue, then baked at 350°F until meringue is golden.