Put together an ice tub in a big bowl. Place 1 quart barista-blend oat milk, the seeds of two vanilla beans (or 1 tablespoon vanilla extract), and 1/8 teaspoon kosher salt in a medium saucepan and whisk to mix. Clip a sweet thermometer to the saucepan and set it over medium warmth. (Alternatively, you’ll be able to verify the temperature with an instant-read thermometer.) Cook dinner, stirring commonly till the combination is steaming and bubbles on the sides, 5 to 7 minutes. In the meantime, place 6 massive egg yolks, 2/3 cup granulated sugar, and 1/4 cup cornstarch in a medium bowl and whisk till combination is pale yellow.