1. Combine tamarind paste, olive oil, chipotle powder, garlic, onion, cilantro, and salt and pepper to taste in a food processor and blend into a smooth paste.
2. Place the chicken thighs in a medium bowl and pour in the marinade, massaging it into the chicken.
3. Marinate in the fridge for at least a couple of hours, ideally overnight.
4. Preheat the oven to 350°F.
5. Heat the olive oil in a cast-iron skillet over medium-high heat.
6. Remove the chicken thighs from the marinade and add them to the hot skillet, searing for 3 to 4 minutes on each side.
7. Move the skillet to the oven to finish cooking the chicken, another 10 to 15 minutes, depending on the thickness of the thighs.
8. Serve right away, with additional cilantro if you like.