Combine flour, 2/3 cup granulated sugar, salt and nutmeg in a large bowl; add buttermilk, lemon rind, lemon juice, melted butter and egg yolks. Stir with a whisk until mixture is smooth. Beat egg whites at high speed until foamy. Add ¼ cup granulated sugar a little at a time until stiff peaks form. Gently stir one fourth of egg white mixture into buttermilk mixture. Gently fold in remaining egg white mixture. Fold in blackberries. Pour batter into an 8-inch square baking pan coated with cooking spray. Place in a larger baking pan; add hot water to larger pan to depth of 1 inch. Bake at 350 for 35 minutes until cake springs back when touched lightly in center. Sprinkle cake with powdered sugar. Serve warm. Serves 6.