5 large egg yolks
¾ cup sugar
1½ cups heavy cream
½ cup 1% or 2% milk
1 cup buttermilk
1 teaspoon pure vanilla extract
Make the base:
In a medium heatproof bowl, whisk the yolks just to break them up, then whisk in half of the sugar (6 tablespoons). Set aside.
In a heavy stainless steel pan, stir together the cream, milk, and the remaining sugar (6 tablespoons) and put the pan over medium-high heat. When the mixture approaches a bare simmer, reduce the heat to medium.
Carefully scoop out about ½ cup of the hot cream mixture and, whisking the eggs constantly, add the cream to the bowl with the egg yolks. Repeat, adding another ½ cup of the hot cream to the bowl with the yolks. Returning to the pan of cream on the stove, use a heatproof spatular to stir the cream as you slowly pour the egg and cream mixture from the bowl back to the pan.
Continue to cook the mixture carefully over medium heat, stirring constantly, until the mixture is thickened, coats the back of a spatula, and leaves a clear mark when you run your finger across it, 1 to 2 minutes longer.
Strain the base through a fine-mesh strainer and into a clean container. Set the bowl into an ice bath, wash your spatula, and use it to stir the base occasionally until it is cool. Then cover with plastic wrap and refrigerate the base for at least 2 hours or overnight. It’s particularly important that the base is cold before proceeding to the nest step; otherwise the buttermilk will cause the mixture to “break” and lose its emulsion.
Freeze the ice cream:
Add the buttermilk and vanilla to the cold base and whisk to blend.
Freeze in your ice cream machine according to the manufacturer’s instructions. While the ice cream is churning, put the container you’ll use to store the ice cream into the freezer. Enjoy right away or, for a firmer ice cream, freeze for at least 4 hours.