Combine milk and sugar in a heavy saucepan. Split the vanilla bean in half lengthwise and scrape out the seeds.
Place the seeds and bean in the saucepan.
Heat mixture over medium heat just until it begins to boil (Do not let it keep boiling).
Remove from heat, cover, and let stand for 15 minutes.
Place egg yolks in a medium bowl and whisk together.
Pour the milk mixture in a steady stream into the yolks, whisking constantly to prevent the eggs from cooking.
Pour the entire mixture back into the saucepan and place back over medium heat. Cook, stirring constantly with a wooden spoon (make sure to get the bottom and sides) until the mixture reaches 185 degrees F and it has thickened enough to coat the back of the spoon.Strain mixture into a clean container.
Add in the heavy cream and stir to combine. Chill mixture in refrigerator for at least 8 hours, preferably 24.Freeze in an ice cream maker per manufacturer's instructions. While the ice cream is freezing, melt the bittersweet chocolate in a double boiler or in a bowl set over a saucepan of simmering water.When the ice cream is almost done (it should be mostly solid), pour in the melted chocolate in a steady stream into the ice cream.
Let it churn for a few minutes for the chocolate to incorporate and shatter into streaks.
Transfer ice cream to a clean container and place in freezer until fully frozen, about 2-3 more hours.
