PREHEAT oven to 350°F.
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. STIR in vanilla.
Meanwhile, BAKE phyllo shells on baking sheets 3 minutes or until slightly crisp. COOL on wire rack.
FILL each shell with about 1 Tbsp. chocolate filling.
BEAT egg whites in medium bowl with electric mixer on high speed until foamy. STIR in cream of tartar.
Gradually ADD remaining 1/4 cup of the sugar, beating until stiff peaks form.
SPREAD meringue over top of each tart, making sure to seal edges.
BAKE tarts on baking sheets 5 to 7 minutes or until lightly browned.
SERVE warm or refrigerate before serving.
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.