Place the potatoes into a medium-sized pot. Add enough water to cover the potatoes and bring to a boil over medium-high heat. Reduce the heat to medium and cook until the potatoes are tender when pierced with a fork, about 20 minutes. Add the cabbage to the pot during the last five minutes of cooking.

Drain the vegetables and return them to the pot. Add the heavy cream and butter and mash with a potato masher or a handheld mixer. Season with salt and pepper to taste.