Heat half-and-half, 6 tablespoons sugar, and salt in medium heavy-bottomed saucepan over medium heat until simmering, stirring occasionally to dissolve sugar. If using vanilla bean, add this as well.
Whisk yolks in medium bowl until thoroughly combined. Whisk in remaining 2 tablespoons sugar and whisk until sugar begins to dissolve and mixture is creamy. Whisk in cornstarch until combined and mixture is pal yellow and thick.
When half-and-half mixture reaches full simmer, gradually whisk half-and-half mixture into yolk mixture to temper. Return mixture to pan, scraping bowl with rubber spatula.
Return to simmer over medium heat, whisking constantly, until a few bubbles burst on the surface and the mixture is thickened and glossy. This takes about a minute.
Off heat, whisk in butter and vanilla (if using vanilla bean, scrape the pod and add seeds). Strain the pastry cream through a fine-mesh sieve set over medium bowl. This will remove any curdled bits that might have formed during the cooking process.
Press plastic wrap directly on surface of pastry cream to prevent skimming and refrigerate until cold and ready to use.