Slice potatoes in half lengthwise. Place on a large microwave-safe dish. Microwave on HIGH, covered with potatoes to the center. 11 minutes. Remove from the microwave and chop potatoes easily with a fork. Pour 1/2 cup milk over the potatoes. Add remaining 3 cups milk, whisking until smooth. Slowly add butter to potatoes while whisking. Microwave on HIGH 8-11 minutes or until hot. Add grated cheese and mix until melted. Serve with toppings. Optional toppings: sour cream, chives, cooked bacon, steamed broccoli florets.