In a medium bowl, whisk the egg yolks and sugar together until pale yellow; set aside.

Combine the cream and milk in a medium-sized heavy bottom sauce pan. Cook over medium heat, whisking occasionally, until the mixture just starts to bubble around the edges of the pan. Remove from heat and place the chopped chocolate in the pan. Stir gently until the chocolate has melted.

Slowly drizzle the chocolate into the egg mixture, whisking constantly until combined. Transfer hot mixture back into the saucepan.

Cook over medium heat, whisking constantly, until the mixture is thick enough to coat the back of a spoon and registers 170 degrees F on a candy thermometer. Strain the mixture with a fine mesh sieve into a medium bowl. Stir in the vanilla and chocolate liqueur.

Place the bowl over an ice bath to cool, whisking frequently to lower the temperature. Cover and refrigerate about 4-6 hours or until completely chilled.

When chilled, process the mixture in your ice cream maker according to the directions from the manufacturer. Transfer to a resealable container and freeze over night.