Preheat the oven to 450 degrees F and place a rack in the middle. Place potatoes directly on the oven rack and bake for 50-60 minutes, until extremely soft when pressed with tongs.
Just before removing the potatoes from the oven, combine the cream and 6 Tablespoons of the butter in a small saucepan set over medium heat. Bring to a simmer, stirring occasionally.
Remove the potatoes and, right away, split each one in half lengthwise. Carefully scoop the tender flesh out of each potato half and place in the bowl of an electric mixer. If there are bits of tough flesh near the skin, leave it behind. Once all the potatoes are scooped out, briefly mix the flesh with a fork, then use the paddle attachment to beat on low speed until they are very smooth. Do all of this while the potatoes are still hot.
Pour the simmering cream mixture into the potatoes. Use a large spatula to gently fold in the cream until the entire mixture is smooth and light. At first it will seem like it’s not coming together, but keep folding and it will happen. Stir in salt, then sample and add more salt to taste.
Transfer potatoes to either a serving bowl or the bowl of your slow cooker, cover tightly with plastic wrap, and refrigerate for up to 2 days.
Slow cooker warming option. Remove plastic wrap and warm potatoes with the extra 2 Tablespoons of butter on top. This depends on exactly how hot your slow cooker gets, but I find low heat for 3-4 hours or high heat for 1-2 hours does the trick. Stir occasionally and add a splash of cream if desired to lighten the consistency. Serve warm straight from the slow cooker.
Microwave warming option. Use a sharp knife to poke 8-10 holes in the top of the plastic wrap, then microwave for about 10 minutes at 75% power, stirring halfway through, until the potatoes are hot. Top with remaining 2 Tablespoons butter and serve hot.