Chill a small mixing bowl. Combine egg yolks, milk, 1 teaspoon cornstarch, and 3 tablespoons sugar in a heavy small saucepan. Cook and stir over medium heat until slightly thickened and bubbly. Cook and stir 2 minutes more. Remove from heat. Stir in the grated baking bar and vanilla; stir until melted. Cool to room temperature.
Beat whipping cream in the chilled bowl on low speed until soft peaks form (tips curl). Gently fold the cooled baking bar mixture into the whipped cream. Spoon the mixture into a 2-quart square baking dish. Refrigerate 4 to 24 hours or until set.
For cherry sauce, combine 1/2 cup sugar and 2 tablespoons cornstarch in saucepan. Stir in water. Add cherries. Cook and stir over medium heat until thickened and bubbly. Cook and stir 2 minutes more. Remove from heat. Add orange liqueur or orange juice. Cool slightly.
To serve, spoon chocolate mixture onto dessert plates. Drizzle with warm cherry sauce.