In a medium-sized saucepan, bring the half-and-half and vanilla to a gentle simmer over medium heat. Meanwhile, vigorously whisk together the egg yolks and sugar in a large bowl.
Once the cream is simmering, turn the heat to low. Slowly pour the liquid into the yolks while whisking to temper the eggs.
Pour the mixture back into the saucepan, and continue stirring in a figure 8 motion until the sauce thickens slightly and coats the back of the spatula, around 3-5 minutes. Remove from the heat and stir in the buttermilk.
Transfer to a clean bowl and cover, placing plastic film directly against the surface of the liquid to prevent a skin from forming, and chill until the mixture is cold, then prepare in an ice cream maker according to manufacturer instructions.
This recipe will work with either whole or park-skim buttermilk. Make sure to shake the bottle before using if the liquid appears separated. I do not recommend using homemade buttermilk for this recipe, which curdles. Use a commercial version instead, which is more stable and smooth.
Please note that this recipe yields more than one quart, so if you have a small machine, you may need to churn the ice cream in two batches.