In a mixing bowl, whisk together 2 tablespoons olive oil, 1 teaspoon chili powder, and the juice of one lime; season with a pinch of salt.
Place the chicken breasts in the bowl and coat them with the marinade; cover and refrigerate for 30 minutes.
Preheat a grill to high heat. Grill the chicken until cooked through, about 10 minutes per side depending on thickness, until internal temperature reaches 165°F (74°C).
Alternatively, heat a large skillet over medium-high heat with 1 tablespoon oil and sear the marinated chicken until browned and cooked through; then remove and rest.
Transfer the cooked chicken to a cutting board, let rest 5 minutes, then thinly slice.
Meanwhile, in a small bowl combine 1/2 cup sour cream, 2 tablespoons chopped cilantro, 1/2 teaspoon ground cumin, and the juice of the remaining lime; stir until smooth to make the cilantro crema.
Heat a nonstick skillet over medium and warm the corn tortillas one at a time, about 10 seconds per side, until pliable and lightly toasted.
Assemble the tacos by filling each warm tortilla with sliced chicken, diced avocado, and diced tomato; drizzle with cilantro crema and top with salsa and extra cilantro if desired.