Add potatoes to a large pot and fill pot with water, just enough to cover the potatoes. Salt the water with a tablespoon of salt. Cover and bring water to a boil. Once boiling, remove cover and cook for another 15 to 20 minutes, or until potatoes are very tender.
While the potatoes boil, add 3 tablespoons of butter to a small sauce pan. Place over medium heat and cook until butter browns, but doesn't burn. Remove from heat and add sage, remaining butter, and heavy cream. Set aside.
Remove potatoes from water (but do not discard the water!) and place into bowl of stand mixer. Add butter and cream mixture. Pulse mixer with paddle attachment to mash the potatoes. If you don't have a stand mixer, simply use a potato masher to achieve this.
Once potatoes are mashed, add 1 or 2 teaspoons of salt (to taste). Mix on low to combine, or use a wooden spoon to combine. If potatoes are too dry, add a little of the reserved potato water until desired consistency is reached. Place in serving dish and serve immediately.