Place the potatoes in a large pot and cover with water. Bring to a boil and cook until the potatoes are tender. Drain and return to the pot.
Add 2 tablespoons of the butter to the potatoes, as well as the milk, rosemary, parsley, salt and pepper. Mash with a potato masher until desired consistency. Crumble in about 3/4 of the blue cheese and mix to combine.
Meanwhile, melt the remaining 2 tablespoons of butter in a small skillet. Continue to cook over medium heat, stirring constantly, until the butter turns a golden brown, 6-8 minutes. Add in the walnuts and cook for an additional minute, making sure the butter doesn't burn.
Transfer the mashed potatoes to a serving bowl. Pour the butter and walnut mixture over the top of the potatoes. Crumble the remaining blue cheese over the top.