Melt the butter in a large pot over medium heat. Add the sliced leeks and garlic, and cook until softened, stirring occasionally, about 5 minutes.

Add the diced potatoes, chicken or vegetable broth, bay leaf, salt, and black pepper to the pot. Bring the mixture to a simmer, then reduce the heat to low and cover the pot.

Cook the soup until the potatoes are tender, about 15 to 20 minutes.

Remove the bay leaf and discard it. Use an immersion blender to purée the soup until it is smooth and creamy. Alternatively, you can transfer the soup in batches to a blender and blend until smooth, then return it to the pot.

If the soup is too thick, you can add milk to thin it out to your desired consistency. Adjust the seasoning with more salt and pepper, if needed.

Serve the soup hot with crusty bread or crackers.

Enjoy your delicious and comforting bowl of Potato Leek Soup!