Melt the butter in a large, heavy pot or Dutch oven over medium-high heat. Add the onions, celery, salt, and cayenne and cook, stirring, until the onions are soft and lightly golden, about 5 minutes. Add the bay leaf and garlic and cook, stirring, for 2 minutes. Add the broth and potatoes and bring to a boil. Reduce the heat to medium-low and simmer, uncovered, until the potatoes are soft, about 30 minutes. 

Remove the soup from the heat. Discard the bay leaf. With a hand-held immersion blender, or in a food processor or regular blender in batches, process until smooth. Slowly add the cream and stir to blend. To serve, ladle into soup bowls and serve hot.