Make the simple syrup and set aside. Heat a small, dry skillet over high heat and add the cumin seeds. Toast the seeds until fragrant, swirling the seeds occasionally so they toast evenly, 30 to 45 seconds. Transfer the hot seeds to a mortar and pestle or a spice grinder and grind to a fine powder. Quickly add the toasted cumin to the simple syrup. Stir and let sit, covered, for 5 minutes.