2 cups heavy whipping cream
1/2 cup whole milk
1/4 cup buttermilk
1/4 cup mascarpone cheese*
4 coffee beans
1 teaspoon finely grated lemon peel
1 vanilla bean, split lengthwise
4 large egg yolks
1/2 cup sugar
2 tablespoons fresh lemon juice
1/4 cup limoncello or other lemon liqueur plus additional for serving
Whisk first 6 ingredients in large saucepan. Scrape in seeds from vanilla bean; add bean. Cook over medium-high heat until bubbles form at edges, stirring occasionally. Remove from heat, cover, and let steep 15 minutes.
Whisk egg yolks, sugar, and lemon juice in large bowl until thick and smooth. Slowly whisk warm cream mixture into yolk mixture. Return custard to same saucepan. Stir over medium heat until custard thickens enough to coat spoon and thermometer registers 180°F, about 6 minutes; do not boil. Pour custard into medium bowl. Stir in 1/4 cup limoncello. Chill custard until cold, stirring occasionally, at least 4 hours. DO AHEAD: Can be made 1 day ahead. Cover and keep chilled.
Process custard in ice cream maker according to instructions. Transfer to container; cover and freeze up to 2 days. S coop gelato into glasses. Drizzle with additional limoncello and serve. * An Italian cream cheese; sold at many supermarkets and at Italian markets