In medium size pot, mix together sugar, cornstarch and salt. Whisk in milk.
Cook and whisk often over medium heat till thickened and bubbly.
In small bowl, whisk together egg yolks. Drizzle small amount of hot pudding mixture into egg yolks, whisking rapidly. Then return pot to heat and slowly drizzle all egg yolk mixture into pot, continuing to whisk quickly. Cook 2 minutes more.
Remove from heat and add butter and vanilla.
Place fine mesh strainer over bowl and pour hot pudding through the strainer to remove any small pieces of egg that may have cooked.
Cover pudding with plastic wrap, placing the plastic directly on the surface of the pudding (this prevents a skin from forming).
Chill completely (or eat warm if you can't wait!)
Top with whipped cream