Preheat your grill to 375°F/190°C (if using a pellet grill), to low heat (if using gas), or for indirect cooking (if using charcoal), with the lid closed, for 15 minutes.
Prick the sweet potatoes all over with a fork and rub them all over with the olive oil. Season generously with salt and pepper. Grill the sweet potatoes over indirect heat, covered, turning occasionally, until very soft inside, about 45 minutes. For extra char, you can move them to direct heat for the final 10 minutes of cooking.
In a small nonstick pan over medium heat, toast the semi-crushed cumin seeds, the pepitas and sesame seeds until fragrant, about 5 minutes. Add the butter to the pan and let it melt, shaking the pan occasionally. Stir in the cilantro and remove the pan from the heat.
To serve, cut a slit down the center of each sweet potato and top with spoonfuls of the cumin seed butter and lots of flaky salt. Serve immediately.