Put the potatoes and garlic (if using) in a medium saucepan and cover by 1 inch with cold water. Season generously with salt. Bring to a boil. Cook at a gentle boil until the potatoes are very tender when poked with a fork, about 10-15 minutes.
Drain the potatoes, shaking out any excess water. Immediately pass the potatoes and garlic through a ricer (in batches) back into the saucepan.
Add the butter, and stir until mostly melted. Stir in 1 cup of milk. If desired, add more milk until you reach the thickness you like. Season with salt to taste (be generous—it makes all the difference!). Put the saucepan back over medium heat and cook until warmed through.
Transfer the pomme puree to a serving bowl or to plates. If you’d like, top with another pat of butter. Serve warm.