Bring a large pot of salted water to a boil over a high flame.
Add potatoes and cook 20-25 minutes, until easily pierced with a fork.
Meanwhile, in a large sauté pan, melt 2 tablespoons butter over a medium flame. Add mushrooms with a pinch of sea salt; and stir. Continue to cook, while stirring, about 6 minutes, until lightly browned.
Add cream or milk and simmer over a low flame until reduced by half, about 10 minutes. When potatoes are done, drain and return to the same pot over a low flame. Stir rapidly for 4 minutes.
Remove from heat and add mushroom and cream mixture along with remaining butter and a pinch of sea salt and pepper.
Mash together until desired consistency is achieved.