Wash the clams in several soaks of water and scrub to remove all sand and grit. Place the clams in the freezer for about 10 minutes before cooking.
Melt the butter in a large saucepan. Add the onions and saute them gently until they're translucent. Remove them from the heat.
Cover the bottom of a large steamer pot with water and bring it just to a boil. Add the clams and simmer, covering the pot. The clams will open in several minutes. Remove each clam just as soon as it opens, discarding any that don't open. Reserve the clam juice in the pot. Remove the clams from their shells and dice them coarsely. Remove any black necks or intestines from the clams. Reserve the clams.
Meanwhile, wash the potatoes and dice them into small cubes. Place them in a pot and add enough water to just cover the potatoes. Salt the water lightly and cook the potatoes until tender.
Drain the potato water into the reserved clam juice and add the diced potatoes to the reserved clams. Remove the kernels from the corn cobs, and then add the kernels to the clams and potatoes.
Add 2 cups of the cream to the chopped onions in the saucepan and return it to a low heat. Stir, and then add the combined clam juice and potato water. Add the cornstarch to the remaining 1 cup cream and stir until it is totally dissolved. Then add this mixture gradually to the onions, raising the heat and stirring constantly. Add the combined clam, potato and corn mixture to the same saucepan and continue to stir until the chowder is nicely thickened. Add salt and pepper to taste. If using, serve with oyster or common crackers.
A viewer or guest of the show, who may not be a professional cook, provided this recipe. It has not been tested for home use.