Peel the potatoes and cut them into equal pieces. Add the potatoes into a large pot, pour over cold water, salt the water. Cook the potatoes until fork-tender.
Mash the potatoes while still hot (do not mash them for too long).
Finely chop rosemary needles, add with the butter into a small pot. Cook for a couple of minutes until the butter is browned - it will smell nutty and have a light brown color.
Add all the other ingredients to the bowl with mashed potatoes: browned butter with rosemary, hot milk, sour cream, garlic powder, and shredded cheddar cheese (on the large holes of a box grater).
Stir until all the ingredients are well-combined. Season to taste with salt and pepper.