Make the pastry cream: Beat the yolks and egg with the sugar and cornstarch until lightened and fluffy. Heat the milk to a simmer and remove from heat. Stream the milk into the egg mixture while whisking. Bring the whole mixture to a simmer, whisking constantly. You will see it thicken. Remove from heat, add the vanilla, butter and salt, and strain through a mesh strainer to remove any clumps. Cover with plastic wrap right on the surface of the mixture to avoid a skin forming. Chill for 4 hours.