Place a steamer insert into a saucepan, and fill with water to just below the bottom of the steamer. Cover, and bring the water to a boil over high heat. Add cauliflower and parsnips, recover, and steam until soft, about 15 minutes.
Drain the vegetables. Mash using a hand masher or electric mixer. Gradually add milk and butter and mash to desired consistency. Season to taste with salt and pepper; serve warm.