In a heavy saucepan combine half-and-half, 3/4 cup sugar, and vanilla bean and bring just to a boil. Remove pan from heat.

In a bowl whisk together egg yolks and remaining 1/4 cup sugar and add hot half-and-half mixture in a stream, whisking.

Return custard to pan and cook over moderately low heat, stirring, until 170°F on a candy thermometer. Remove pan from heat.

Scrape seeds form vanilla bean into custard and discard pod. Stir sour cream into custard until combined well and strain through a fine sieve into a bowl.

Chill custard until cold and freeze in an ice-cream maker. Transfer soft ice cream to freezer-proof container and freeze until hard, approximately 1-2 hours. Makes about 1 quart.