Place the chicken breast on a cutting board and cover with a piece of plastic wrap. Use a rolling pin or mallet to briefly pound the chicken breast to an even thickness (this should only take 15-20 seconds). This will help it cook quickly and evenly without drying out.
Combine the chili powder, cumin, garlic powder, and salt in a small bowl. Sprinkle the spice mixture over both sides of the chicken breast.
Heat the cooking oil in a skillet over medium heat. Once hot, add the chicken breast and cook on each side until cooked through (about 5 minutes each side, use a thermometer to make sure the internal temperature reaches 165ºF). Allow the chicken to cool for 5 minutes on a clean cutting board, then chop into tiny pieces.
While the chicken is cooking, roughly chop the spinach into smaller pieces. Add the chopped chicken, spinach, cheese, and sour cream in a bowl and stir until evenly combined.
Divide the filling between four tortillas, then fold the tortillas closed. Place the quesadillas (one or two at a time) in a skillet* over medium heat. Cook on each side until the tortillas are brown and crispy, and the filling is melted (3-5 minutes each side). Slice each quesadilla in half and serve.