Combine chocolate and butter in a large heavy saucepan. Cook and stir over low heat until melted. Remove from heat, stir in liqueur and cool slightly.
Preheat oven to 350 degrees. Beat egg whites in a large mixing bowl at high speed with an electric beater until stiff peaks form. Beat egg yolks and sugar in another large mixing bowl at medium speed until thick and pale. Fold one-third of chocolate mixture into yolk mixture. Gently fold in one-third of egg whites. Fold in remaining chocolate mixture and egg whites. Pour batter into a greased and floured 10-inch spring form pan.
Bake at 350 degrees for 30 minutes or until edges are set (center will not be set). Remove from oven, and gently run a knife around edge of pan to release torte. Cool to room temperature on a wire rack; when cooled, cover and chill in refrigerator for 8 hours. Remove sides of pan just before serving. Serve with heated chocolate sauce and berries, if desired.