Melt chocolate in saucepan. Combine sugar, milk and egg yolks. Add to chocolate and cook until smooth and thick, stirring constantly with wire whisk. Remove from heat. Cream butter and add 3/4 cup powdered sugar. Add chocolate mixture and mix well. Let cool slightly. Beat egg whites with salt until stiff. Beat remaining powdered sugar into egg whites. Fold into chocolate mixture, add vanilla. Cut sponge layers in half and cut to fit bottom of spring-form pan. Line sides with ladyfingers. Fill with chocolate mixture and chill several hours or overnight. Top with whipped cream and chocolate shavings.