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Preheat grill to medium-low.
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Meanwhile, toast peppercorns and mustard seeds (if using) in a small skillet for 2-4 minutes or until fragrant. Let cool slightly and then place in a food processor, spice mill, or small blender. Add cumin, paprika, brown sugar, salt, oregano, chili powder, celery seeds, garlic powder, and cayenne paper (if using) and blend until ground.
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Rub spice mixture all over and inside chicken (you might not require it all*). Place the chicken on top of the open, half full beer can, so that the can is in the cavity.
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Place beer can/chicken on top of a drip pan (I used a tin pan) and place on grill. Cook, with grill closed, for 50-60 minutes (or until an thermometer reads 165° F when inserted into the meaty thigh).
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Remove from grill and let rest for 5 minutes before carving.