Place potatoes in a large pot, add water to cover the potatoes by 2 inches, salt well (the water should taste like seawater), and bring to a boil over high heat. Reduce heat to low and simmer potatoes until fork tender, about 15 to 20 minutes.
Drain potatoes, return to the pot along with butter, and cook over low heat until butter is melted. Using a hand-held potato masher, smash potatoes until butter is incorporated, leaving some lumps. Add cream, truffle oil, salt, and pepper and fold together. Taste and adjust seasoning as desired.