In a saucepan, combine the sugar, cornstarch and salt. Gradually add milk and cream until smooth. Cook and stir over medium-high heat until thickened and bubbly, about 2 minutes. Add a small amount to egg yolks; mix well. Return all to the pan. Bring to a gentle boil; cook for 2 minutes, stirring constantly. Remove from the heat; stir in butter and vanilla.