Place potatoes in a large heavy pot and cover with water. Add 1 teaspoon of salt and bring to a rapid boil. Cook potatoes until tender, about 15 to 20 minutes.
Meanwhile, infuse the milk by adding milk, cream, 8 tablespoons of butter, and fresh rosemary sprigs to a medium saucepan, over medium heat. Simmer for 5 to 6 minutes, just until the butter melts. Turn off the heat, and steep for 15 to 20 minutes, or longer. Strain the milk mixture and set aside until ready to use.
To make the brown butter, add 4 tablespoons of butter to a small saucepan over medium heat. Cook until brown, bubbly, and little specks on the bottom of the pan (approximately 5-6 minutes or longer, depending on the stovetop). Set aside until ready to serve.
Drain the potatoes and place back in the saucepan. Cover with a dish towel and allow the steam to evaporate any excess water for 5 minutes. Remove the towel, press the potatoes through a ricer, or mash with a potato masher until creamy and smooth. Squeeze in the roasted garlic, and add the infused milk mixture and the cream cheese or mascarpone, parmesan cheese, ½ teaspoon salt, a pinch of black pepper, or more to taste, and stir to combine. Cover to keep warm.
