1. Begin by partially cooking the potatoes. Put them in a saucepan and cover them with water. Bring the water to a boil. Reduce the heat until the potatoes are just simmering. Allow them to simmer about 3 minutes; they'll still be firm. Immediately plunge them into cold water to stop the cooking, then drain and set aside.
2. In a large, heavy-bottomed stock pot, melt the butter. Add the salt pork and sauté over low heat until it turns light brown (5-7 minutes).
3. Add the onions and continue sautéing over low heat until translucent (4—5 minutes).
4. making the roux: Roux is a cooked combination of flour and butter that will act as the thickening agent when you add the liquid ingredients. To make the roux, add the flour to the butter, salt pork and onions. Cook the roux for 5 minutes, stirring constantly; otherwise your soup will have a floury taste. Pay attention to the heat. Burning the roux is worse than not cooking it at all, as a burned roux will give your soups a scorched flavor.
5. While making the roux, warm the clam juice in a saucepan.
6. Slowly ladle the warm clam juice into the stock pot, stirring constantly. Increase the heat to medium. Keep stirring until you've blended the roux and the clam juice completely and the mixture is thick and smooth.
7. Add the chopped clams, corn, and continue to cook over medium heat for 5 minutes.
8. Add the cream (both kinds) and potatoes. Season with salt and pepper. Reduce the heat until the chowder is just simmering. Let it simmer over low heat for 15 minutes, stirring occasionally to prevent the ingredients from sticking to the bottom of the pot.
9. Serve immediately.