Add the potatoes and garlic to a large saucepan, cover with cold water and add 2 large pinches of salt. Bring to a boil, reduce to a simmer and cook until a fork can easily pierce the flesh, 20 to 25 minutes.
Meanwhile, heat the cream and milk in a small saucepan until just simmering. Cut the heat, stir in the butter to melt and season with a big pinch of salt and pepper.
Drain the potatoes in a colander. Add them back to the pot they were cooked in and place over low heat to get rid of excess water. Once dry, turn off the heat and mash the potatoes with a wooden spoon. Stir in the warm cream mixture and the truffle cheese and Parmesan. Season with salt and pepper.