In a medium bowl, whisk together the egg yolks, sugar and salt. In a medium saucepan, heat the cream and buttermilk over medium heat, stirring occasionally, until almost simmering, about 5 minutes. Add to the egg mixture in a slow stream, whisking constantly, then pour into the saucepan. Cook over medium-low heat, stirring constantly, until the custard thickens and registers 175° on an instant-read thermometer, about 8 minutes.