Prep and cook the potatoes. Peel the potatoes and cut them into chunks. Place them in a 4 qt pot and cover them with cold water. Season the water liberally with salt. Bring to a simmer over medium heat and boil the potatoes until they’re very tender and easily pierced with a fork.
Make the garlic herb butter. While the potatoes cook, melt the butter in a small saucepan over medium heat. Once the butter is melted, add the smashed garlic cloves and chopped herbs and cook for 2 minutes, stirring constantly. Add the cream and bring to a very gentle simmer. Turn off the heat and let the cream steep for 10 minutes, then discard the garlic cloves.
Mash the potatoes. Drain the potatoes in a colander and return them to the pot they were cooked in. Lightly mash the potatoes, then add the warmed cream and butter mixture. Continue to mash the potatoes and add the mascarpone. Mash to your desired consistency, then season the mashed potatoes to taste with salt and pepper and serve right away.