Steam potatoes and cauliflower. Using a vegetable steamer, add 1-2″ water to bottom of pot. Insert steamer. Add veggies. Bring to boil and steam until fork-tender, 20-25 minutes. Drain.
In mixer bowl with paddle attachment, beat potato cauliflower mixture on low speed until smooth, 2 to 3 minutes.
Add butter, salt and pepper. Mix until well blended. Do not overmix.