Sprinkle garam masala, chili powder, coriander, cumin, ginger and salt over potatoes; stir to combine. Sprinkle cornstarch over mixture; stir to combine. Add peas, 1 tablespoon of the cilantro, and the jalapeño chile. Mix completely. In small bowl, mix remaining tablespoon cilantro with the yogurt; stir to blend, and set aside.