Put the potatoes in a large pot and boil until soft (about 20 minutes).  Use a colander to strain off the water and return the potatoes to the pot.  Add the cream cheese and sour cream.  Use an electric hand mixer to mash and blend the potatoes.  Season to taste using season salt and pepper.  Add the baking soda and thoroughly blend.  Place in a buttered baking dish and dot the top with butter.  Cover with foil (you can put the dish into the refrigerator at this point and it will keep just fine for several days).  Preheat the oven to 325 degrees.  Bake for 15 minutes leaving the foil covering in place, then remove the foil and bake for another 20 minutes.