Heat oil in large saute pan over medium-high heat. Add onion; season with salt to taste. Cook for 5 to 6 minutes, stirring occasionally, until softened, then add the butter and 3 tablespoons curry powder. Season with pepper to taste, stirring to combine.
Stir in chickpeas, 1 1/4 cups vegetable broth and brown sugar. Once the mixture has begun to bubble at edges, reduce heat to medium and cook for 10 minutes. Taste and adjust seasonings as needed, adding some or all of the remaining 1 tablespoon curry powder.
Whisk together the remaining 1/4 cup vegetable broth with cornstarch in a measuring cup, stirring to dissolve the cornstarch. Add to the chickpea filling. Increase heat to medium-high so the broth comes to a boil. When the liquid has thickened, remove from heat and stir in the chopped cilantro. Transfer to 8-by-11-inch baking dish.