Peel and thinly slice potatoes on mandoline, or by hand carefully to ensure consistent thickness.
Place potatoes in a saucepan and cover with heavy cream. Add a good couple pinches of salt. Bring to a gentle boil over medium heat until just barely fork tender.
While potatoes are cooking, cut the dark green tops from the leeks. Discard or set aside for stock making. Slice leek lengthwise then slice into half moons, about 1/4" thick. Rinse and pat dry.
Wash and dry kale, then remove leaves from the thick stalks. Roughly chop.
Heat butter in large saute pan over medium low heat. When butter is melted add leeks and a pinch of salt. Saute gently until softened, stirring occasionally to prevent charring.
When potatoes are mostly soft, pour into a colander placed over a bowl to catch the cream.
Spread the potatoes out on platter or sheet pan to cool and return cream to pan. Simmer on low until reduced by half, season with salt and pepper to taste.
Add chopped kale to the leeks and stir to coat kale with the oil. Cover with lid to steam.
Once kale as softened, remove lid and saute until tender. Taste and adjust salt as needed.
When cream has reduced, layer half the potatoes in an 9x13 pan. Top with kale and leek mixture. Pour over half the cream. Layer remaining potatoes and top with the rest of the cream.
Bake for 20 - 25 minutes, or until top is golden and bubbly. Add chopped parsley if desired and serve.