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.
06 - Taste and adjust seasoning as needed. Serve hot, with an extra pat of butter melting on top.