Simmer 3 pounds Yukon Gold potatoes (peeled and cut into chunks) with 3 bay leaves in salted water until tender, 15 to 18 minutes. Reserve 1/2 cup cooking water, then drain; discard the bay leaves. Return the potatoes to the pot off the heat and mash. Meanwhile, bring 1 1/2 cups milk to a simmer in a saucepan with 5 chopped garlic cloves and 3 bay leaves; cook 15 minutes. Remove from the heat. Add 1 3/4 sticks cut-up butter to the potatoes along with the milk mixture (discard the bay leaves); stir, thinning with the reserved cooking water if needed. Season with salt and pepper.