Peel your potatoes and cut them into large cubes. Warm up the butter in a pressure cooker or a soup pot. Lightly sauté the potato cubes in butter. Add the broth until the potatoes are mostly submerged, usually half a quart or ~2cups.
Cook the potatoes and the broth on pressure for 7 minutes. Allow the pan to depressurize. (If you don't have a pressure cooker, just boil the potatoes in the broth until soft, about 35 mins)
Stir the watercress leaves into the potato broth mixture, and boil the leaves for 1 minute. Remove from heat. Let the soup cool.
Pour the milk, adding milk until it reaches the desired creaminess. If you have an immersion blender, you can leave the soup in the same pot in which you cooked it. If you are using a blender, don't be afraid to blend in two batches.