Scrub potatoes. Leave skins on, if desired, or peel thinly and remove eyes. Cut into large pieces. Heat 1 inch water (salted if desired) to boiling in 3-quart saucepan; add potato pieces. Cover and heat to boiling; reduce heat. Cook covered 20 to 25 minutes or until tender; drain. Shake pan gently over low heat to dry potatoes.
Heat milk, whipping cream, butter, salt and pepper in 1-quart saucepan over medium-low heat, stirring occasionally, until butter is melted. Measure out 1/4 cup of the milk mixture; cover and refrigerate.
Add remaining milk mixture in small amounts to potatoes in medium bowl, mashing with potato masher or electric mixer after each addition. (Amount of milk needed to make potatoes smooth and fluffy depends on kind of potatoes used.) Beat vigorously until potatoes are light and fluffy.
Spray 2-quart casserole with cooking spray. Spoon potatoes into casserole. Bake immediately, or cover and refrigerate up to 24 hours.
Heat oven to 350ºF. Pour reserved milk mixture over potatoes. Bake uncovered 40 to 45 minutes or until potatoes are hot. Just before serving, stir potatoes.