In a large pot, bring potatoes to a boil in salted water over high, then reduce to a rapid simmer. Add leek and kale and cook until potatoes are tender when pierced with a knife, about 10 minutes. Drain; transfer to a large bowl.
In a large cast-iron or nonstick skillet, cook bacon over medium-high until crisp, 8 to 10 minutes. With a slotted spoon, transfer to bowl with vegetables. Season with salt and pepper. With a fork, mash potato mixture until a few lumps remain. With your hands, form into 12 patties.
Pour off fat from skillet and wipe clean; add butter and melt over medium. In batches, cook cakes until golden on both sides, about 6 minutes, flipping once.