1. In a medium saucepan, whisk together sugar, cornstarch, cocoa, and salt. Whisk in milk and egg yolk. Bring to a boil over medium-high, whisking constantly; cook 2 minutes. Remove from heat; whisk in vanilla and chopped chocolate until melted. Pour mixture into a bowl and cover with plastic wrap, pressing wrap directly onto surface to prevent skin from forming. Refrigerate until cold, at least 30 minutes and up to overnight.