Place the 2½ pounds yukon gold potatoes in a large pot and cover with cold, generously salted water. Bring to a boil, then simmer for 15–20 minutes, or until fork-tender. Drain well.
2½ pounds yukon gold potatoes
Return the hot potatoes to the pot and cook over low heat for 1–2 minutes, stirring gently, to evaporate excess moisture. Mash using a potato masher or ricer until smooth.
Meanwhile, add the 5 tbsp unsalted butter, 1 cup heavy cream, 2 cloves garlic, and 1 sprig rosemary to a sauce pot. Bring to a simmer on medium heat.
5 tbsp unsalted butter, 1 cup heavy cream, 2 cloves garlic, 1 sprig rosemary
Stir in the heavy cream mixture into the potatoes until smooth and combined. Slowly pour in the warm cream, stirring until creamy and smooth.
Add the 8 oz brie cheese pieces and stir gently until fully melted and incorporated. The potatoes should be silky and rich.
8 oz brie cheese
Stir in 1 1/2 tsp white truffle oil, starting with 1 teaspoon and adding more to taste. Season with 1 tsp salt and ½ tsp black pepper.
1 1/2 tsp white truffle oil, ½ tsp black pepper, 1 tsp salt
Transfer to a serving bowl, top with flaky salt and an extra pat of butter, and serve warm.