Preheat the oven to 400°F. Pierce the potatoes several times with a fork. Place in the microwave and use your baked potato setting until the potatoes are cooked through, turning half way.
Cut the potatoes in half lengthwise. Scoop the flesh out into the large bowl with the cauliflower, leaving a ¼” shell. Place the potato shells on a baking sheet.
Mash the flesh with a potato masher or puree with a hand blender along with the cauliflower. Add the buttermilk, sour cream, butter, pepper, and salt, mash until smooth.