Mash or rice the potatoes. Add in the butter and continue Mashing or ricing them until butter is absorbed. Then add half-and-half (or milk). Mash or rice them until you reach the consistency you like.
If you are not serving the potatoes immediately, let the mashed potatoes cool completely and refrigerate in an airtight container in the refrigerator for up to 5 days or freeze up to 3 months.
To Reheat the mashed potatoes in the oven, place in a covered dish at 350° F for about 20-25 minutes, or until the potatoes are completely heated through.