Peel and wash the potatoes, then cut them into evenly sized chunks.
Place the potatoes in a large pot of salted water and bring to a boil.
Cook until the potatoes are fork-tender, about 20–25 minutes
Drain the potatoes and mash until smooth
While the potatoes cook, wash the spinach thoroughly and finely chop it
Melt the butter in a skillet over medium heat
Add the spinach and cook for 2–3 minutes until wilted
Warm the milk in a small saucepan
Add the spinach mixture, warm milk, and salt to the mashed potatoes
Stir continuously over low heat until fully combined
Beat or whip the mash for 1–2 minutes until light, fluffy, and creamy
Serve immediately