Place the diced potatoes in a pot with the stock.
Bring to a boil, then cover, reduce to a simmer, and steam for about 15 minutes or until potatoes are fork tender.
Drain, and reserve the remaining broth.
Transfer the potatoes to a large bowl.
Roughly mash the potatoes using a potato masher.
Add the butter and Greek yoghurt.
If the potatoes seem dry, add some of the reserved vegetable broth a couple of tablespoons at a time.
Season with salt and pepper to taste and serve.