In a medium bowl, whisk together the egg yolks and sugar until very pale yellow and smooth.
In a medium saucepan, bring to a boil the heavy cream, sour cream, and vanilla bean with its scrapings.
Whisk about half this mixture into the egg yolk mixture until well combined, then pour back into the saucepan. Over medium heat, stirring constantly with a wooden spoon, cook until the mixture coats the back of the spoon. (It is very important to stir constantly. Do not overcook; cooking too long will result in scrambled eggs.)
Pour through a fine-mesh strainer into a clean bowl and immediately set the bowl over a larger bowl filled with ice cubes and cold water until chilled, stirring occasionally, about 30 minutes. Stir in the liqueur. Refrigerate, covered, until needed.
Through step 3, the crème anglaise can be made up to 3 days ahead.
