• In a large pot over medium heat, melt butter. Cook leeks in butter with salt and pepper until tender, stirring frequently, about 15 minutes.
• Stir cornstarch into broth and pour broth into pot. Add the potatoes and bring to a boil. Season with salt and pepper. Pour in the cream, reduce heat and simmer at least 30 minutes, until potatoes are tender.
• Season with salt and pepper before serving. Serve in bowls and garnish each bowl with cheese, bacon (optional) and green onions.

Tips:

• Fill a good sized bowl with cold water, drop the leeks in as you're slicing them. This will allow them to soak and the dirt will settle in the bottom of the bowl. You may have to rub the dirt off of some parts of the leeks. This is the best way to clean leeks because if you do it under running water, it takes a lot longer but also wastes a lot of water.
• I slice the leeks the same way I do a green onion (only bigger) and then lay them down flat and slice in half.
• I whisk the cornstarch in about 1-2 cups of chicken broth, that way you don't need to use a huge bowl for all of the chicken broth.
• Temper the heavy cream so it doesn't curdle (it will still taste the same but I don't like the texture when this happens). You can use the same bowl you used to whisk the cornstarch and chicken broth. Take about 1 or 2 cups of the soup and place into the bowl. Add 1 cup of the heavy cream mix and pour back into your stockpot. Repeat.
• During step 3, half way through the 30 minute simmer, I take my potato masher and lightly mash some of the potatoes at the bottom of my stock pot. This makes the soup a little thicker but be careful don't mash all of the potatoes, you want to leave chunks of potato in the soup to give it texture.