Preheat oven to 325°.
Combine 1/2 cup sugar and 2 tablespoons water in a small, heavy saucepan. Cook over medium-high heat until the sugar dissolves, and continue cooking 5 minutes or until golden brown (do not stir). Immediately pour into 6 (6-ounce) custard cups or ramekins coated with cooking spray, tipping quickly until caramelized sugar coats bottoms of cups.
Combine 2/3 cup sugar, salt, eggs, and egg yolks in a medium bowl, stirring well with a whisk.
Heat milk over medium-high heat in a small, heavy saucepan to 180° or until tiny bubbles form around edge (do not boil). Gradually add hot milk to egg mixture, stirring with a whisk. Stir in liqueur. Strain mixture through a sieve into a bowl, and discard solids. Divide mixture evenly among prepared custard cups. Place cups in the bottom of a broiler pan; add hot water to pan to a depth of 1 inch. Bake at 325° for 40 minutes or until flan centers barely move when custard cups are touched. Remove cups from pan; cool completely on a wire rack. Cover and chill at least 8 hours.
Loosen edges of flans with a knife. Place a dessert plate, upside down, on top of each cup, and invert onto plates.