Stir together flour and sugar in a medium mixing bowl. Using a pastry blender, cut in the 1/2 cup cold butter until pieces are the size of small peas. Stir together 2 egg yolks and 1 tablespoon water in a small mixing bowl. Gradually stir egg yolk mixture into flour mixture. Using your fingers, gently knead the dough just until a ball forms. If necessary, cover with plastic wrap and chill for 30 to 60 minutes or until dough is easy to handle.
Preheat oven to 450 degree F.
Shape pastry dough into thirty-six 3/4-inch balls. Evenly press the balls into the bottoms and up the sides of ungreased 1-3/4-inch muffin cups. Bake in the preheated oven for 6 to 8 minutes or until edges are lightly browned. Cool in pans on a wire rack. Remove pastry shells from muffin cups; set aside.
Meanwhile, for filling, heat and stir the semisweet or bittersweet chocolate, the 1/4 cup whipping cream and the 3 tablespoons butter in a heavy, medium saucepan over low heat just until chocolate is melted. Gradually stir about half of the chocolate mixture into the 1 beaten egg yolk. Return entire egg yolk mixture to the saucepan. Cook and stir just until mixture begins to bubble; remove from heat.
Stir the desired liqueur or 2 tablespoons whipping cream into chocolate mixture in saucepan. Transfer chocolate mixture to a medium mixing bowl. Refrigerate for 1-1/2 to 2 hours or until mixture is completely cooled and smooth, stirring occasionally. (Butter may separate out; stirring will blend it in.)
Beat chilled chocolate mixture with an electric mixer on medium speed about 2 minutes or until light and fluffy. Spoon chocolate mixture into a decorating bag fitted with a large star tip (about 1/2-inch opening). Pipe mixture into the pastry shells. If desired, drizzle with melted white chocolate.
Lightly cover tartlets; chill for 6 to 24 hours. Before serving, let stand at room temperature for 15 to 20 minutes. Makes 36 tartlets.