Wash and dry lemons and finely rub the peel off one. Squeeze out both. Whip the condensed milk in a bowl with the juice, zest, flavor and citric acid until creamy. Whip the cream until stiff and fold into the lemon mixture. Put everything in a container and place in the freezer for about 4 - 5 hours. For longer storage, take the container about 15-20 minutes before removing the desired portions from the freezer.