Split vanilla beans lengthwise with a sharp paring knife. Gently scrape out all the seeds.
Place vanilla-bean scrapings and pods in a medium saucepan with milk and 1/2 cup sugar. Scald the mixture, cover, and remove from heat. Let steep for 30 minutes.
Whisk together egg yolks and remaining sugar in a medium heatproof bowl.
Return milk to stove and bring just to a simmer. Using a measuring cup or ladle, slowly pour about 1/2 cup of the hot-milk mixture at a time into the egg-yolk mixture, whisking constantly until blended.
Pour mixture back into saucepan, and cook over low heat, stirring constantly, until mixture is thick enough to coat the back of a spoon, 3 to 5 minutes. Add cream.
Pour through a fine sieve into a heatproof bowl set in ice bath. Let cool, stirring occasionally.
Place plastic wrap on surface of custard to prevent a skin from forming, and refrigerate 2 hours or overnight.
Freeze in an ice-cream maker, according to the manufacturer's instructions.