In a 5-quart saucepan, combine the potatoes, 2 tablespoons butter, the garlic, 1 teaspoon salt and enough cold water to cover. Bring to a boil over medium-high heat. Reduce the heat to a simmer and cook until the potatoes are tender, 15 to 20 minutes. Drain in a colander and remove the garlic cloves. Return the potatoes to the pan and, using a potato masher, mash the potatoes until smooth.
Add the mascarpone, Parmesan, chicken broth, the remaining 4 tablespoons butter, 2 teaspoons salt and 1 teaspoon pepper to the potatoes. Stir over low heat until smooth and warmed through. Season with salt and pepper. Transfer to a bowl and serve.