Wash, peel and roughly dice the potatoes and cook in salted water for 10 minutes. Wash the broccoli, cut into florets, cut the stalk into slices, add to the potatoes, cook everything together for another 10 minutes. Drain, let steam off a little. Put some broccoli florets aside for the garnish.
Mash the broccoli and potato mixture with the potato masher. Heat milk with butter, gradually stir in. Add sesame seeds, reserve some for sprinkling. Season to taste with salt, pepper and wasabi.
Serve sprinkled with sesame seeds and garnished with broccoli florets.