Place the potatoes and parsnips in a large pot and cover with water. Add 2 tablespoons of kosher salt. Bring to a boil. Cook for 20 minutes until the potatoes and parsnips are fork-tender. Drain and return to the pot. Let stand for 5 minutes.

Turn the heat on the pot to low. Add the mascarpone, whole milk, and vanilla paste. Use an immersion blender to blend until just smooth, adding more milk if needed, until the purée reaches your desired consistency. Be careful not to overwork the potatoes. Add 2 teaspoons of kosher salt and stir to combine. Taste and add more salt if desired.

Spoon the purée onto a serving platter and finish with freshly minced chives. Enjoy!