Place the unpeeled potatoes in a large saucepan and cover with cold water. Add a pinch of salt and bring to the boil, then reduce the heat and simmer for 25–30 minutes. (Test the potatoes by piercing them with a sharp knife. If they are ready, the blade will come out clean with no residue.) Drain well.
Peel the potatoes while they are still hot, using a tea towel to protect your hands. Pass the peeled potatoes through a fine sieve into the same saucepan. Use a wooden spoon to stir the mash until all the moisture is removed (you want the potato to be dry to the touch).
Bring the milk to the boil, then reduce the heat to a simmer. Place the mash potato over low heat and add 50 g of butter, stirring until combined. Add 50 ml of milk and stir until combined. Repeat until all of the butter and milk has been added and the mash is creamy and light. Season with salt to taste.
Place the mash on a serving plate and use a dessertspoon to achieve a wavy effect, as shown. Serve immediately.