If needed, place the insert of the ice cream maker in the freezer to freeze for at least 8 hours.

Prepare an ice bath in a large bowl. Place 1 quart barista-blend oat milk, the seeds of 2 vanilla beans (or 1 tablespoon vanilla extract), and ⅛ teaspoon kosher salt in a medium saucepan and whisk to combine. Clip a candy thermometer to the saucepan and set it over medium heat. (Alternatively, you can check the temperature with an instant-read thermometer.) Cook, stirring regularly until the mixture is steaming and bubbles on the edges, 5 to 7 minutes. Meanwhile, place 6 large egg yolks, ⅔ cup granulated sugar, and ¼ cup cornstarch in a medium bowl and whisk until mixture is pale yellow.

Once the milk mixture is hot, slowly ladle about ½ cup into the egg mixture while whisking constantly to temper it. Pour the tempered eggs into the saucepan and cook, stirring constantly, until thick enough to coat the back of a wooden spoon and 165ºF, 7 to 8 minutes.

Place the saucepan in the ice bath, being careful not to let any water get into the pan. Let cool for 20 minutes.

Pour the custard through a fine-mesh strainer into a storage container and discard the contents of the strainer. Cover the custard and refrigerate until completely cool, at least 2 hours but preferably overnight.

Transfer the ice cream base to the bowl of the ice cream machine. Churn until the base has thickened to a soft-serve ice cream consistency. In most ice cream makers, this takes about 20 minutes — check the instructions for your particular machine.

Transfer the churned ice cream to a freezer container. Press a piece of wax paper against the surface of the ice cream to prevent ice crystals from forming. Freeze until solid, at least 8 hours. Let the ice cream sit at room temperature for 5 to 10 minutes to soften slightly before scooping.