Preheat oven to 400 degrees. Spray a rimmed baking sheet with cooking spray or oil mister.
Steam potatoes and cauliflower in a steamer basket over a few inches of boiling water for about 20 minutes, or until easily pierced with fork. Transfer to large bowl and mash thoroughly--if you're piping these, it's important to make sure the potato and cauliflower mixture is completely lump-free! It's best to use a potato ricer if you have one.
Add cheese, egg, milk, yogurt, salt and pepper to potatoes and cauliflower. Use a hand mixer on low speed to combine ingredients; fold in chives.
Transfer mashed potato mixture to piping bag or a large plastic zip-top bag. Since the potatoes will lose their shape anyway, it's best just to use a large round tip. If you're using a plastic bag, cut off the corner and pipe the potatoes through that. I piped in a spiral, starting from the outer-edge and working my way towards the center moving clockwise, but again, since these end up flattening out a little, it doesn't matter too much how you do it.
Spray tops of potatoes with oil mister or cooking spray. Bake for 15 minutes, then turn on broiler and broil for about 5 minutes or until edges begin to brown.