In a small bowl, stir together the yogurt and cornstarch until well blended.
In a small (1-quart) saucepan, melt the butter over medium heat. Add the onion, sprinkle with a pinch of salt, and cook, stirring frequently, until softened, 4 to 5 minutes. Add the garlic and ginger and cook, stirring frequently, until just golden-brown, 4 to 5 minutes more (reduce the heat if the onion seems to be burning rather than browning). Add the curry powder and cumin and cook, stirring, 15 to 20 seconds. Reduce the heat to mediumlow, add the yogurt mixture, and stir until slightly thickened, about 1 minute. Season with 1/4 tsp. each salt and pepper, or to taste.
Drizzle the sauce over steamed vegetables and sprinkle with the cilantro, if using.