1. Cook cream in a heavy nonaluminum saucepan over medium heat, stirring often, 4 to 5 minutes or just until bubbles appear (do not boil); remove from heat.
2. Whisk together egg yolks and next 2 ingredients in a medium saucepan. Slowly whisk hot cream into egg yolk mixture.
3. Cook over low heat, whisking constantly, 10 to 15 minutes or until mixture thickens and coats the back of a spoon. Remove from heat, and cool completely (about 30 minutes).
4. Whisk in lemon zest, lemon juice, and buttermilk.