In a large saucepan, whisk the sugar, cornstarch, cream, and milk together until smooth. Place over medium heat and stir occasionally until bubbles begin to form and the liquid thickens.

In a medium bowl, whisk the egg yolk. Carefully, while continuously whisking, pour in 1/2 cup of the milk mixture. Whisk until smooth and pour back into the saucepan.

Return the saucepan over medium heat and bring to a simmer, always stirring. Let simmer and thicken for one minute Remover from heat.

Off heat, stir in the vanilla and butter until smooth. Cover with plastic wrap - pressing the wrap directly over the pudding so that it covers the surface, touching it, and chill for 4 hours before serving.