Peel potatoes and cut into even pieces (you can boil them whole, but cutting them increases the surface area and cooks them much quicker).
Place into a pot with rosemary and a generous pinch of salt, cover with cold water, cover with a lid, and bring to a boil. Once boiling, reduce to a simmer and cook for 10-20 minutes. They are ready when you can easily pierce a piece of potato with a knife.
Pour off 1-2 cups of the cooking liquid into a measuring cup and set aside. Drain the rest of the potatoes and remove rosemary.
Add the butter and a generous pinch of salt to the potatoes and mash with a potato masher until everything is uniform. The hot potatoes should melt the butter easily. While mashing, add in reserved cooking water a bit at a time until a smooth consistency is reached (for 6 potatoes, you will end up using about 1 cup, though this will depend on the type of potatoes you use and how starchy they are)
Once everything has been mashed, taste for seasoning, then use a hand mixer or immersion blender with a whisk attachment to whip the potatoes until silky smooth. DO NOT use a regular immersion blender blade - this will turn your mashed potatoes into glue!
Top with some melted butter, chopped herbs of your choice and enjoy!