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Combine the guajillo chiles and water in a small bowl and let stand 1 hour.
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Remove chiles, reserving liquid and remove seeds and stems.
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Place in a blender along with garlic, onion, piloncillo and ½ cup reserved liquid. Blend until smooth.
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Pour marinade over steaks in baking dish. Cover tightly with plastic wrap and refrigerate overnight.
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About 20 minutes before grilling, remove the steaks from the refrigerator and let sit, covered, at room temperature.
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Prepare barbecue (medium-high heat). Remove steaks from marinade and grill to desired doneness, about 4 minutes per side for medium-rare.
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Transfer the steaks to a cutting board or platter, tent loosely with foil and let rest 5 minutes before slicing.