How to Make It
In a large saucepan, cover the potatoes with water and bring to a boil. Add a generous pinch of salt and simmer over moderate heat until tender, about 20 minutes. Drain the potatoes in a colander, shaking off any excess water.
In the same saucepan, melt the butter in the milk over moderately low heat. Remove from the heat and stir in the cheese. Press the potatoes through a ricer into the pot and mix well. Fold in the mayonnaise and jalapeño and season with salt and white pepper. Serve right away.