Add salt, sugar, vinegar, garlic and onion powder and cayenne into a large stock pot.
Pour in 8 cups of water and whisk to combine. Heat on medium, stirring ocassionally until the salt and sugar have dissolved.
Remove off of the heat and carefully add 9 to 10 cups of ice. Stir until all the ice has melted and brine is cool.
Place pork shoulder (do not trim off the fat) in brining buckets and carefully pour the brine over top. Secure the lid and refrigerate for 12 hours.
Meanwhile prep your smoker by lining the bottom and the top of the smoke box with foil. Fill an can (top portion removed with can opener) with water. Measure out 6 ounce of hardwood (hickory, cherry or apple for example).
FOR THE RUB:
After 12 hours, remove the pork from the brine and rinse with water. Pat well with paper towel. Set the pork out on your counter for 30 to 40 minutes to take the chill off.
Then rub the pork generously with yellow mustard (no precise measurement).
And with a heavy hand, massage the rub all over the pork. Be sure to get all the sides completely covered and even in the nooks and crannies. Just when you think you've put on enough rub, add more. (this is my husband's rule)
TO SMOKE THE PORK SHOULDER:
Place the rubbed pork shoulder fat cap side down. Insert the probe through the middle on the side of the pork shoulder (avoiding the bone), until it reaches the center of the meat.
Set the smoker temp to 225° and the probe (food temp) to 195°.
Once the probe temperature reads 160°, open the smoker and carefully remove the pork shoulder and place on a baking sheet with 2 pieces of heavy duty aluminum foil.
Double wrap the pork shoulder to avoid any additional smoke to get to the meat. Be sure to wrap the foil around the probe. Place the pork back into the smoker until the probe (food temp) reads 195°.
When the pork reaches temperature, remove and place into a cooler and pack with towels (do not use your best towels or cooler here!). Allow the pork to rest for 2 to 4 hours before removing and pulling/shredding.
Serve on buns with bbq sauce and sides of your choice.