Place potatoes in a large saucepan and fill with cold water to cover potatoes. Add a pinch of salt. Bring to a boil, reduce heat and simmer for about 15 min. or until potatoes are tender. Drain and put back into saucepan.
Add butter, milk and chopped jalapeno pepper, smash, or roughly mash, potatoes and mix well to combine.
Spread potato mixture in 13 x 9-in (3 L) baking dish. Top with mozzarella cheese, place under a broiler and heat until cheese is melted and slightly bubbly.