Use best-quality ingredients when preparing a batch of this simple, classic flavor. Martha made this recipe on Martha Bakes episode 604.
Combine cream, milk, 6 tablespoons sugar, and salt in a medium saucepan over medium heat. Scrape vanilla seeds into pan; add pod. Heat until hot (do not let boil), about 2 minutes. Remove from heat; cover and let stand 30 minutes.
Prepare an ice-water bath. Whisk together egg yolks and remaining 6 tablespoons sugar in a medium bowl. While whisking constantly, add a ladle of hot cream mixture in a slow stream, then add another ladle. Pour yolk-mixture into remaining hot cream mixture in pan. Cook over medium heat, stirring constantly, until custard is thick enough to coat the back of a spoon and registers 170 degrees on an instant-read thermometer, 5 to 7 minutes. (The custard should retain a line drawn with your finger across the back of a spoon.)
Strain through a fine sieve into a bowl set in the prepared ice-water bath. Let stand until cold, stirring occasionally.
Pour mixture into ice cream maker. Process according to manufacturer's instructions. The ice cream is ready once it holds its shape when the machine stops but sags slightly. Transfer to an airtight container and freeze.