Separate egg yolks from egg whites, one by one, and remove any white membrane clinging to the yolk. After separation, place each egg white in a container with a lid and freeze for later use in meringue or for macaroons.
In a heavy saucepan, combine half-and-half, 3/4 cup sugar and vanilla bean and heat just to a boil.
Remove pan from heat.
In a bowl, whisk together egg yolks and remaining 1/4 cup sugar.
Add the hot half-and-half mixture in a steady slow stream, whisking until it is all incorporated.
Return the mixture to pan and cook over moderately low heat, stirring until it reaches 170 degrees on a candy thermometer.
Remove pan from heat again. Scrape the seeds from vanilla bean into mixture until combined well. Discard the pod.
Stir sour cream into this custard until well combined and then strain through a fine sieve into a large bowl.
Chill custard until cold and freeze in an ice cream maker.