In a large saucepan, whisk together sugar, cocoa powder, and cornstarch. Slowly whisk in the milk until the powders are dissolved. Whisk in the heavy cream and egg yolk, then place over medium heat.
Stir often until the mixture comes to a boil. Reduce heat to a simmer and continue to cook and stir until thickened. Remove from heat. Mix in the Irish cream, dark chocolate, butter, and vanilla extract until smooth.
Allow to cool slightly, then divide among individual serving glasses. Cover the tops with plastic wrap, making sure it is touching the tops of the pudding to keep skin from forming. Refrigerate to chill thoroughly, at least 2-4 hours to overnight.
Serve chilled topped with whipped cream and grated chocolate.