Stracciatella Gelato

Place milk in a medium saucepan. Scrape in seeds from vanilla bean; add bean. Bring to a simmer, whisking often. Whisk sugar, yolks, and salt in a medium bowl until well blended, about 1 minute. Gradually whisk hot milk mixture into yolk mixture. Return to saucepan; stir over medium-low heat until custard thickens and a thermometer registers 170 °F-175 °F, about 5 minutes (do not boil). Strain into a medium bowl. Stir in cream. Chill custard until cold, at least 2 hours. Stir chocolate and oil in a small sauce-pan over low heat until melted; let cool. Process custard in an ice cream maker according to manufacturer's instructions. Slowly add all but 1 tsp. melted chocolate during the last 30 seconds of churning (chocolate will form small chips). Transfer to a bowl. Drizzle remaining chocolate over gelato in zigzag lines. Serve immediately.