01 - Place potatoes in a large pot, cover with cold salted water, and bring to a boil over high heat. Reduce heat and simmer for 15–20 minutes, or until potatoes are fork-tender.
02 - While potatoes cook, melt 2 tbsp butter in a large skillet over medium heat. Add kale and sauté for 4–5 minutes until tender and bright green. Stir in the green onions and cook for 1 minute longer. Remove from heat.
03 - Drain potatoes and return to the pot. Mash thoroughly until smooth.
04 - Gently heat milk and cream (if using) in a small saucepan until just warm.
05 - Add the sautéed kale, green onions, warm milk (and cream if using), remaining butter, salt, and pepper to the mashed potatoes. Stir until well combined and creamy. Taste and adjust seasoning as needed.
06 - Serve hot, with an extra pat of butter melting on top.