Prepare the smoker: Preheat your pellet grill or smoker to 225℉ with your choice of wood or pellets. *Sometimes I'll cook my briskets at 235℉ to speed up the cook time just a bit.
Trim the brisket: Use a sharp knife to trim the excess fat from the brisket, leaving about ¼" of the fat cap. Place the fat pieces in a disposable foil pan to make the smoked tallow.
Season the brisket: Rub the hot sauce all over the brisket on both sides. Season liberally all over with the fajita or Mexican all-purpose seasoning. The hot sauce acts as the binder.
Smoke the beef: Insert a meat thermometer probe into the middle of the brisket. Place the brisket on the top rack of the smoker fat side up with the point side facing your heat source. Place the pan with the fat pieces on the bottom grate. Smoke until the internal temperature of the brisket reaches 180°F (about 8 hours for us, but cooking times will vary).
Wrap the smoked brisket: Remove the beef from the smoker and place the brisket on top of 2 large sheets of butcher paper (about 48" long each) that are slightly overlapped. Pour about 1 cup of the melted smoked tallow over the top of the brisket (optional). Wrap the brisket in the butcher paper, then return the wrapped brisket to the smoker with the fat side up. Cook until the internal temperature reaches 200 to 205°F.
Rest the brisket: Remove the brisket from the smoker and rest the brisket in the paper until the internal temperature reduces to about 150°F.
Rehydrate the dried chiles: While the brisket is resting, make the sauce. Cut the stems off the dried guajillo peppers and remove the seeds. Add the peppers to a pan of water about 2" deep, set over medium-high heat. Bring to a boil, then turn off the heat and let the peppers sit in the hot water for about 15 to 20 minutes to steep.
Blend the sauce: Remove the peppers from the water, and add them to a blender with the chipotle pepper and about ¼ cup of the water from the pan where you were steeping the peppers. Blend until the mixture is smooth.
Finish the brisket: Once it’s cooled down, remove the brisket from the paper. Cut into slices against the grain, and then chop the meat and add it to a large bowl or pan. Pour the sauce over the top, and toss to coat the beef all over. Add additional seasoning to taste, if desired.
Use the chopped brisket to make tacos, nachos, burritos, and more!