For the leek cream soup, halve the leek lengthwise and cut into 1 cm long pieces. Cut a small piece of leek into fine strips and set these aside.
Peel the potato and cut into small cubes.
Heat the butter and sauté the leek pieces and potato cubes in it. Pour in soup and simmer gently for about 20 minutes. Grind with a hand blender.
Add whipped cream and milk and boil up again. Season to taste with salt and pepper.
Serve the cream of leek soup in soup bowls, decorated with a dollop of whipped cream and a few leek strips.