Using 2 tbsp (28 g) of the butter, grease the bottom and 2 inches (5 cm) up the sides of the cooking pot.
Tie the sprigs of thyme and rosemary together with kitchen twine, then add the bundle to the cooking pot along with the cooking potatoes, garlic, 1 cup (235 ml) milk, salt, and black pepper.
Cover with the lid, press Slow Cook, adjust it to High, then set the timer to 5 Hours. Cook until the cooking potatoes are very tender, 4 to 5 Hours. Press Cancel.
Remove and discard the herb bundle. Add the remaining 1 tbsp (14 g) butter and mash the cooking potatoes (and garlic) in the cooking pot, adding more milk if needed, 1 tbsp (15 ml) at a time, to reach the desired consistency.
Spoon the mashed cooking potatoes into a serving bowl and sprinkle with chives (if using)