Prepare and boil the potatoes. Wash and peel the potatoes, then cut them into approximately 2- to 3-inch cubes, keeping the pieces as even in size as possible. Place the potatoes in a large pot and add enough cool water to completely cover them. Add 1 tablespoon of salt, then bring to a boil over medium-high heat. Cook until the potatoes are fork-tender but not falling apart. Drain well.
Infuse the butter and cream. While the potatoes are boiling, add the butter, smashed garlic cloves, rosemary, thyme, heavy cream, and milk to a small saucepan. Bring to a very gentle simmer over low heat, allowing the garlic and herbs to infuse the mixture as it warms. Do not allow the cream to boil.
Mash the potatoes. Remove the rosemary, thyme, and garlic from the warm butter and cream mixture. Using a potato masher, mash the drained potatoes while gradually adding the warm butter and cream mixture until the potatoes reach your desired consistency.
Season and serve. Season the mashed potatoes to taste with salt and black pepper. Serve warm.