Preheat oven to 375 degrees F. Prick each potato a few times with a fork. Bake 60 to 70 minutes, or until quite tender. Cook the broccoli in boiling water until just tender, about 2 minutes. Cool in ice-cold water, drain well, and then coarsely chop.
When the potatoes are done, cool until safe enough to handle, but still quite warm. Cut off the top third of the each potato. Carefully scoop out as much potato flesh as you can and place it in a bowl. Place the hollowed out potato shells in a baking dish lined with parchment paper. Mash the potatoes until smooth; whip in the butter, sour cream and milk. Mix in the broccoli, green onion, salt, pepper and three-quarters of the cheese. Spoon, pack and mound the mixture into the potato shells and sprinkle with remaining cheese. Bake at 375 degrees F for 25 to 30 minutes, or until golden brown.
Note: A broccoli crown is the top portion of the vegetable sold with the stem removed.