Whisk 1⁄2 cup sugar, egg yolks, and cornstarch in medium bowl until smooth. Bring half-and-half, remaining sugar, and salt to simmer over medium heat in large saucepan. Whisk 1⁄2 cup simmering half- and-half mixture into egg yolk mixture. Slowly whisk tempered yolk mixture into saucepan. Cook, whisking constantly, until mixture is thick and large bubbles appear at surface, about 2 minutes. Remove from heat and stir in butter and vanilla.