Wash the potatoes well. Put them in a deep bowl with warm water. Cover with plastic wrap well so there's no air. Put them in the microwave and boil until ready (mine were ready in 10 min).
Take them out and carefully remove the plastic wrap. Arrange them in a baking tray, Salt, sprinkle with black pepper, a little olive oil and crumble/tear the rosemary.
Put a piece of butter onto each potato. Bake in a preheated 392°F (200°C) oven to a golden crisp.
Take them out and while still warm, make a couple cuts on top in an X shape. Grab the ends with both fingers and squish the top part.
Serve with a lump of butter. You can also add 1 tsp sour cream inside the potato opening.