Preheat oven to 425°F. Line a large baking sheet with parchment paper or foil for easy cleanup. Set aside.
Spray a 6-quart (or larger) slow cooker with non-stick cooking spray. Trim brisket of excess fat, rinse, and pat dry. If the brisket is too long, cut it in half to fit in the slow cooker.
In a medium bowl, whisk together all Spice Rub ingredients. Remove 1 tablespoon of the rub and reserve it for the barbecue sauce. Rub the remaining spice mixture evenly all over the brisket, then place the brisket on the prepared baking sheet. Let the brisket sit at room temperature for 30 minutes or refrigerate up to 24 hours, if desired.
Bake the brisket uncovered for 30 minutes at 425°F to sear the meat.
While the brisket is baking, prepare the barbecue sauce. In a medium bowl, whisk together all Barbecue Sauce ingredients with the reserved Spice Rub.
Remove 1/2 cup of the barbecue sauce and add it to the slow cooker along with 1 cup water, Worcestershire sauce, soy sauce, and beef bouillon. Whisk to combine. Carefully transfer the seared brisket to the slow cooker, ensuring as much of the brisket is submerged in the liquid as possible.
Cover and cook on LOW for 8-10 hours, flipping the brisket halfway through cooking, until the brisket is very tender. If the brisket is tough, it needs to cook longer.
Once tender, remove the brisket from the slow cooker and place it on a foil-lined baking sheet. Brush generously with barbecue sauce and broil for 5-10 minutes until slightly caramelized.
While broiling, heat the remaining barbecue sauce in a microwave or saucepan until warm. Brush the brisket again with the sauce and slice it across the grain before serving. Serve with the extra barbecue sauce and enjoy!