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. Press the potatoes through a ricer into the saucepan and mix well. Fold in the mayonnaise, season with salt and white pepper and serve.