Powdered sugar Whole blackberries FOR SAUCE: Puree berries with sugar, Grand Marnier and lemon juice in blender. Strain sauce through fine sieve to remove seeds. (Can be prepared 1 day ahead. Cover and refrigerate.) FOR CAKE: Position rack in center of oven and preheat to 350 F. Butter 6to 8-cup glass loaf dish. Blend 2/3 cup sugar and flour in large bowl. Mix in lemon juice, butter, lemon peel and egg yolks. Stir in buttermilk. Beat egg whites in medium bowl until soft peaks form. Gradually add 1/4 cup sugar, beating until stiff peaks form. Fold whites into buttermilk mixture. Pour batter into prepared loaf dish. Place dish in large baking pan. Pour enough hot water into baking pan to come halfway up sides of loaf dish. Bake until top of cake is light golden, about 1 hour 10 minutes. Remove loaf dish from water bath. Let cool at least 15 minutes. Dust cake with powdered sugar. Spoon hot or warm cake into deep bowls. Drizzle with sauce and garnish with berries. Pass extra sauce separately.