Makes 3 3/4 cups
Pour the milk and cream into a medium-sized sauce pot. Then use a paring knife to slice open the vanilla bean horizontally. With the back of the knife, scrape the vanilla into a the pot and add the emptied pod as well. Over medium heat, scald this mixture. Tiny bubbles will appear on the edges of the pot and it should be very hot, but not boiling. Remove the pot from the stove.
In a medium sized mixing bowl, use an electric mixer to whip the egg yolks with the sugar. Mix until it's very thick and pale yellow. When you drizzle it with a spoon it should rest for a second or two before sinking back into the rest of the mixture -- this is called the ribbon stage.
Use a ladle to add about half of the hot milk mixture to the eggs and blend with a whisk -- this is called tempering. Now you can add this to the remaining milk mixture in the pot. Stirring almost constantly with a wooden spoon, over medium-low heat, bring it to a strong simmer, about 4 minutes.
Turn the heat to the lowest setting, and gently stir in the chocolate, mixing until it's completely melted. Remove from the heat and strain the sauce. Then stir in the Frangelico.