Step 1 Split the vanilla bean in half lengthwise and scrape the seeds into a non-aluminum saucepan. Add the pod and cream. Gently heat over medium heat until hot, about 4 minutes.
Step 2 Meanwhile, mix the sugar and eggs with an electric mixer until thick and yellow, 1 minute. Stir in some of the hot cream, then return the mixture to the pan. Cook over low heat, stirring constantly, until thickened slightly, about 5 minutes.
Step 3Immediately pour the mixture through a fine strainer to remove any lumps. Press on the vanilla bean to extract all the seeds and flavor. Discard the pod. Stir in the buttermilk. Chill the mixture thoroughly, at least 5 hours.
Step 4Freeze in an ice cream maker according to the manufacturer's directions. Store in a plastic container, covered airtight.
Step 5To serve, let the ice cream rest at room temperature until it softens slightly, about 15 minutes. Serve immediately.