Place buttermilk and softened butter in a microwavable glass bowl; microwave on HIGH until warm, 30 to 60 seconds. Add warm buttermilk mixture, cream cheese, Parmesan cheese, salt, and white pepper to potatoes; mash with a potato masher. Top with chopped fresh chives.
Make it ahead: Prepare 1 day ahead; cover and refrigerate. Reheat before serving.