Combine sugar and whipping cream in a small heavy saucepan, and cook over low heat, stirring constantly, until sugar dissolves.
Beat egg yolks until thick and pale. Gradually stir about one-fourth of hot mixture into yolks; add to remaining hot mixture, stirring constantly. Cook over medium heat, stirring constantly, until thickened. Remove from heat, and stir in liqueur. Pour through a wire-mesh strainer into a bowl; cool.