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. (Or you can bake 1 hour in your oven at 425° F)
Meanwhile, in a medium saucepan, combine cauliflower and broccoli with a little salted water and cook, covered for about 3 minutes.
Remove the broccoli with a slotted spoon and set aside, continue cooking the cauliflower until soft, about 5 more minutes. Drain and set aside in a large bowl.
Preheat the oven to 400°F.
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 and salt, mash until smooth.
Fold in half of the shredded cheddar (and chives if desired).
Spoon the potato/cauliflower mixture back into the shells.
Top with the cooked broccoli (I just inserted them in) and remaining cheese on top.
Bake for 5-10 minutes, or until heated through and the cheese is melted. (you can also microwave it for a few minutes if you prefer not to use the oven)