In a large stainless steel or enameled saucepan, heat the milk until bubbles rise around the edge of the pan.
Remove from the heat, add vanilla, cover, and let steep for 20 minutes.
Meanwhile, melt the chocolate with the whiskey in a heavy saucepan over low heat, stirring occasionally; remove from heat and set aside.
In a big bowl, whisk the egg yolks with the sugar until thoroughly combined; gradually whisk in the warm milk.
Pour this mixture back into the saucepan and heat, stirring constantly, until just slightly thickened; the temperature should be 170° on an instant-read thermometer.
Strain the custard through a fine-mesh strainer into a clean bowl and let cool.
Stir the melted chocolate mixture and cream mixture into the custard; cover and refrigerate until chilled.
Freeze the ice cream in an ice cream maker according to the manufacturer's directions; transfer to a freezer container and freeze for an hour or two to firm before serving.