Place the potatoes in a pot. Cover with water and add the salt. Bring to a boil and cook until tender. Meanwhile, heat the butter in a small skillet and saute the onion, cumin and garlic until the onion is translucent, about 3 to 4 minutes. Add the corn and cook for 2 more minutes. Add the milk and simmer for 1 or 2 minutes, the season with salt and pepper.
Remove the potatoes from the heat and drain. Mash the potatoes and add the corn mixture. Serve hot, garnished with cilantro.