Chocolate Pudding

In a medium saucepan, combine sugar, cocoa powder, cornstarch, and salt. Slowly pour in milk, whisking to combine. Place saucepan over medium heat and whisk until mixture comes to a boil, 6 minutes.  Place egg yolks in a medium heat proof bowl and beat until light and frothy. Slowly pour in about 1/2 cup hot cocoa mixture, whisking to combine. Slowly pour egg mixture back into sauce pan, whisking constantly.  Return to medium heat, whisking, until thickened to a pudding-like consistency, about 3 minutes.  Take off heat and whisk in chocolate, butter, and vanilla until smooth. If mixture looks lumpy at all, strain it through a fine mesh strainer.  Pour into a large bowl and place plastic wrap directly on surface of pudding. Refrigerate until chilled, 2 hours.  When ready to serve, spoon into individual bowls or ramekins and top with whipped cream and chocolate shavings.