Combine heavy cream, whole milk, 1/4 cup sugar, and a pinch of kosher salt in a medium saucepan. Split a vanilla bean lengthwise and scrape in seeds; add pod (or use 2 teaspoon vanilla extract).
Bring mixture just to a simmer, stirring to dissolve sugar. Remove from heat.
If using vanilla bean, cover; let sit 30 minutes.
Whisk 6 large egg yolks and 1/4 cup sugar in a medium bowl until pale, about 2
minutes. Gradually whisk in 1/2 cup warm cream mixture.
Whisk yolk mixture into remaining cream mixture. Cook over medium heat, stirring
constantly, until thick enough to coat a wooden spoon, 2-3 minutes.
Strain custard into a medium bowl set over a bowl of ice water; let cool,
stirring occasionally.
Process custard in an ice cream maker according to manufacturer's
instructions. Transfer to an airtight container; cover. Freeze until firm, at
least 4 hours and up to 1 week.