1. Melt butter in a medium stockpot over medium heat. Add leeks, season generously with Kosher salt, and cook until soft and wilted, about 8 minutes. Stir occasionally to avoid burning. Add thyme and cook for about 30 seconds.
2. Add bay leaf, vegetable stock, and potatoes. Season with Kosher salt and freshly ground pepper. Bring to a boil and then reduce heat to maintain a simmer. Cover and cook for 20-30 minutes, until the potatoes are tender and cooked through. Remove the bay leaf.
3. Using an immersion blender or transferring the mixture to a blender, purée soup until smooth. If using a regular blender, return soup to pot. Stir in heavy cream and cook for about 5-10 minutes more.
4. Ladle soup into bowls and top with a drizzle of parsley oil.

For Parsley Oil

1. Bring a small pot of water to a boil. Blanch parsley for 30 seconds, then transfer the parsley to an ice bath to stop cooking and preserve the bright green color. Puree parsley in a food processor (if it's not pureeing easily, add a couple drops of olive oil to help it along). Place pureed parsley into some cheesecloth (I just used 2 paper towels since I didn't have any), and squeeze parsley so it releases its juice into a small bowl. Whisk in a little bit of olive oil (1/8-1/4 cup).