The mildly spicy, citrusy taste and floral aroma of cardamom brighten the rich flavor of the creamy sweet potatoes in this dish.
Preheat oven to 450 degrees with racks in upper and lower thirds. In a small skillet over medium-high, toast coriander seeds until dark brown and fragrant, about 3 minutes. Grind in a spice grinder, or crush with a mortar and pestle.
In a small saucepan, bring butter, coriander, cardamom seeds and pods, 2 1/2 teaspoons salt, and 1/2 teaspoon pepper to a boil. Toss with potatoes on a rimmed baking sheet to evenly coat. Transfer half of potatoes to a second rimmed baking sheet; spread in a single layer on both sheets.
Roast, flipping potatoes and rotating sheets once halfway through, until potatoes are easily pierced with the tip of a knife and golden brown in places, 30 to 35 minutes. Serve, sprinkled with cilantro.