Double Chocolate Pudding Parfait

Whisk together cornstarch, 1/4 cup sugar, 1/4 cup cocoa, and a pinch of salt in a medium saucepan, then gradually whisk in milk. Bring to a boil, whisking constantly then boil, whisking, until thickened, about 3 minutes. Remove from heat and whisk in chopped chocolate and butter until melted. Transfer pudding to a metal bowl and quick chill by setting in an ice bath and stirring occasionally, about 10 minutes. Meanwhile, beat cream with vanilla and remaining 2 tablespoons of sugar and 1 tablespoon cocoa until it just holds stiff peaks. Layer pudding and cream in 8-ounce glasses. Serve dusted with cocoa. Note: Parfaits can be made 1 hour ahead and chilled, covered.