Be sure to use the best buttermilk you can get for this recipe: The ice cream is just as much about the rich, tangy buttermilk as it is the fragrant peaches. Slideshow: More Ice Creams
Bring a medium saucepan of water to a boil. Fill a large bowl with ice water. Using a sharp paring knife, mark an X on the bottom of each peach. Add the peaches to the saucepan and blanch until the skins start to peel away, 1 to 2 minutes. Transfer the peaches to the ice bath and let cool completely. Wipe out the saucepan.
Peel and chop the peaches. Transfer to a food processor and puree until smooth. Scrape into a large bowl and whisk in the buttermilk, lemon zest and lemon juice. Cover and refrigerate until cold.
In a heatproof medium bowl, whisk the egg yolks. In the medium saucepan, simmer the cream with the sugar, salt and the vanilla bean and seeds over moderate heat, whisking occasionally, until the sugar has dissolved, about 5 minutes. While whisking constantly, slowly stream half of the hot cream mixture into the egg yolks. Pour the mixture back into the saucepan and cook over moderately low heat, whisking constantly, until the custard is thick enough to coat the back of a spoon, 8 to 10 minutes. Strain the custard through a fine sieve set over a heatproof bowl and let cool to room temperature. Whisk in the chilled buttermilk-peach mixture. Press a sheet of plastic wrap directly onto the surface of the custard and refrigerate until very cold, at least 3 hours.
Working in 2 batches, freeze the ice cream base in an ice cream machine according to the manufacturer's instructions. Pack the ice cream into plastic containers and freeze until firm, at least 4 hours or overnight. Serve the ice cream topped with sliced peaches.
The ice cream can be frozen for up to 2 weeks.