In a large covered pot of cold water, bring potatoes to a boil over high heat, then reduce heat to medium & cook about 15 minutes or until potatoes are tender. Drain potatoes & leave them in the pot.
Meantime, in a large frying pan, cook hazelnuts over medium heat, stirring often, until fragrant, about 5 minutes.
Add butter to the nuts & cook about 5 minutes, stirring frequently, until butter is golden brown & flecked with brown bits & hazelnuts are dark brown, then pour hazelnut mixture into a bowl & set aside.
Mash the hot potatoes in the pot, then add milk, yogurt, salt & pepper, mashing to blend.
Transfer to a serving bowl, & spoon about half the warm hazelnut mixture over the top, serving the rest on the side.