Melt butter over medium heat in a large sauce pan. Once butter has melted, add shallots and garlic and cook until soft, but not brown, for about 10 minutes. Season with a pinch of salt. Add the chicken broth and bay leaf and bring to a boil. Add peas and season with salt again. Cook peas for 5 minutes, or until soft. Add the mint and cook for 2 more minutes. Remove the bay leaf and blend with a blender or immersion blender until smooth, adding a little more chicken broth if the soup seems too thick.
Serve immediately with a spoonful of crème fraiche or sour cream in the center of the bowl. Serves 4.