Cook the potatoes: Toss the chopped potatoes, 2 tablespoons of butter, cubed, 1 teaspoon salt and 1/2 teaspoon black pepper in a 5-6 quart slow cooker. Add 1 cup of water, cover, and cook on HIGH until the potatoes are tender, about 4 hours.
Herb Infused Butter: Melt 1/2 cup butter in a small saucepan over low heat. Add the smashed garlic cloves, and sprigs of rosemary and thyme. Let the herbs steep in the melted butter for a while. Remove garlic cloves and the herb sprigs. Set aside.
Mash the Potatoes: Using a potato masher, smash the potatoes until they are mostly smooth. Add the milk and sour cream to the potatoes. Start with 1 cup of milk, adding more as necessary. Then mix in the herb infused butter. Season to taste with additional salt and pepper. Garnish with chives.