In a covered large saucepan cook potatoes in lightly salted boiling water about 20 minutes or until tender; drain and return to saucepan. Add half-and-half and 2 Tbsp. of the butter; cover and let stand 5 minutes. Mash with a potato masher. Season with salt and pepper; cover to keep warm.
Meanwhile, in a large skillet cook the leeks and garlic in the remaining 4 Tbsp. butter over medium heat 5 minutes or until tender. Add parsley and cook 1 to 2 minutes or until wilted. Serve over potatoes.