Place the cauliflower, potatoes and salt in a large pot with just enough cold water to cover. Bring to a boil over high heat, reduce to a simmer over medium-low heat, and cook, uncovered, until tender, about 20 minutes. Drain well in a colander.
Return the mixture to the cooking pot, stream in the half-and-half, and mash with a manual masher until chunky smooth. Stir in the butter, salt and pepper. Taste and adjust seasoning as needed. Serve very hot with additional pats of butter, if desired.
Notes
This can be made ahead and refrigerated, covered, for a day or two and gently reheated over low heat or a water bath.