Put the potatoes, 2 teaspoons salt and enough cold water to cover in a medium saucepan. Bring to a boil, then reduce to a simmer and cook until the potatoes are almost tender, 3 to 6 minutes. Reserve 1 cup cooking water, then drain the potatoes.
Melt the butter in a large skillet over medium-high heat. Add the potatoes and cook, stirring occasionally, until golden, about 8 minutes. Stir in the curry powder and cayenne and cook, stirring, 30 seconds.
Add the chickpeas, 1 cup fried onions and 1/2 cup reserved cooking water; cook, mashing the potatoes with a spoon, until heated through, about 3 minutes. Add more cooking water, if needed. Season with salt.
Mix the yogurt, chopped cilantro, lime juice and 2 tablespoons water in a bowl. Divide the potato-chickpea mixture among bowls. Top with the yogurt sauce, the remaining 1 cup fried onions, the sliced jalapeno and cilantro leaves.