Preheat oven to 350 degrees F (175 degrees C). Lightly grease and flour 9x5 inch loaf pan. Place the dates, sugar, salt, and butter into a bowl, and pour in the boiling water. Stir together and set aside to cool. Once cool, mix in the egg. Stir the flour, baking soda, and cream of tartar together in a bowl. Pour the raisin mixture over the flour mixture, and stir together until thoroughly combined. Mix in the vanilla extract and, if desired, walnuts. Pour batter into prepared loaf pan. Bake in preheated oven until knife inserted in center comes out clean, about 1 hour 15 minutes. Cool in pan 10 minutes before turning out onto a rack to cool completely. Wrap in aluminum foil and store in the refrigerator.