Preheat oven to 350~. Grease and flour 10" springform pan. Melt chocolate chips and butter in top of double boiler over hot, but not boiling, water. Beat yolks in large bowl of electric mixer until very thick, about 5 minutes. Beat in 1/2 cup sugar 1 tablespoon at a time. Stir in melted chocolate, pecans, 1 tablespoon liqueur and vanilla. Beat whites with cream of tartar in another large bowl until soft peaks form. Gradually add remaining 1/2 cup sugar and beat until stiff but not dry. Gently fold 1/4 of whites into chocolate mixture, then fold chocolate mixture back into remaining whites. Pour into prepared pan. Bake 30 minutes. Reduce oven temperature to 275~ and continue baking 30 minutes. Turn off oven; let cake stand in oven for 30 minutes with door ajar. Remove cake from oven. Dampen paper towel and place on top of cake for 5 minutes; remove towel (top of cake will crack and fall). Cool cake in pan. Remove springform. Transfer cake to platter. Beat whipping cream in large bowl of electric mixer until soft peaks form. Beat in powdered sugar and liqueur. Spoon whipped cream onto top of cake and smooth evenly. Sprinkle with chocolate curls if desired. Refrigerate 6 hours. Let stand at room temperature 30 minutes before serving. ----