In a small bowl, stir the garlic powder, smoked paprika, onion powder, chili powder, cumin, and salt together, then rub the spice mixture all over the brisket. Transfer the brisket to your slow cooker.
Next, whisk all of the sauce ingredients EXCEPT the cornstarch and water together in a bowl or glass measuring cup, then pour over the brisket.
Cook on LOW heat for 6-8 hours.
Once the time is up, carefully transfer the brisket to a small baking sheet, then spoon a few spoonfuls of the sauce overtop. Place the brisket in the oven and broil for 3 minutes. Remove from the oven, then repeat this process 1 or 2 more times.
Pour the remaining sauce into a saucepan, then heat over medium until it comes to a simmer.
Next, whisk the cornstarch and water together, then pour the slurry in with the sauce and whisk to combine. Continue simmering the sauce until it thickens, about 5 minutes.
Serve brisket with the sauce on the side, or pour some of the sauce overtop. Slice and enjoy!