Bring a large pot with water to boil over high heat. Add potatoes and cook until tender, about 20 minutes. Drain and return to same pot.
Add remaining ingredients except shiso. Use a masher or a hand mixer to whip potatoes to a smooth consistency, adding more cream if needed. Transfer to a serving bowl and sprinkle with shiso if using.
*Shiso is an aromatic herb with tender, heart-shaped, frilly-edged leaves; find it at East Asian markets.