Steam the potatoes thoroughly. (Medium heat for 30 minutes)
While hot, take them out and mash with a masher.
Add 15g of butter and 30ml of milk to the hot mashed potatoes and mix well.
Add 30g of mayonnaise, 8g of mustard, and salt & pepper to taste, then combine everything together.
About 300g of mashed potatoes are ready! (I usually serve it in two portions.)