In a saucepan, combine sugar, cornstarch, and salt.
Gradually stir in milk.
Bring to a boil over medium.
Boil for 1-2 minutes, stirring constantly.
Stir a small amount of hot filling into egg yolk; return all to pan while stirring constantly.
Bring to a gentle boil; stir for 2 more minutes.
Remove from heat; stir in butter and vanilla.
Cool for 15 minutes.
Fold in whipped cream.
Store in the refrigerator.
Serve with egg nog pound cake.