1. Peel all of the skin off of the potatoes and cut them into cubes. Place the potatoes in a large pot and cover with cold water. Make sure there's about an inch of water above the potatoes.
2. Bring the water to a boil over high heat, then reduce the heat to maintain a gentle boil. Cook the potatoes until they are fork-tender, which usually takes about 15-20 minutes.
3. While the potatoes are cooking, combine the heavy cream and milk in a small saucepan. Heat over low heat until warm but not boiling, (or warm them up in the microwave careful not to bring to a boil).
4. Once the potatoes are cooked, drain them in a colander and let them sit for a minute to let any excess moisture evaporate.
5. Return the potatoes to the pot or transfer them to a large mixing bowl. Use a potato masher, ricer, or an electric hand mixer to mash the potatoes until they reach your desired consistency.
6. Gradually add the warmed cream mixture to the mashed potatoes, stirring or beating until fully incorporated.
7. Stir in the butter pieces, salt, and pepper until the butter is melted and evenly distributed. If you'd like to add sour cream and chives, fold them into the mixture at this point. Adjust the seasoning to your preference.