Sauté the sliced onion in 1 tablespoon of olive oil over medium heat until translucent, approximately 5 minutes.
Add 2 cups of shredded beef brisket, 1 cup of barbecue sauce, 1 teaspoon of garlic powder, 1 teaspoon of salt, and 1 teaspoon of black pepper to the skillet, stirring until heated through for another 5-7 minutes.
Toast the 4 burger buns until golden brown and generously fill each with the brisket mixture, topping with 1 cup of shredded cheddar cheese.
In a bowl, combine the remaining 2 cups of shredded beef brisket with 1 teaspoon of cumin and 1 teaspoon of chili powder.
Warm the 8 small taco shells according to package instructions, about 5 minutes.
Fill each taco shell with the brisket mixture and top generously with salsa, shredded lettuce, another cup of cheddar cheese, and a dollop of sour cream.