Clean and cut the chicken breasts into 20 small pieces. Whisk the yogurt in a small bowl, and then stir in 2 teaspoons oil, the garlic paste, ginger paste, lemon juice, cumin, chili powder, garam masala, turmeric and salt to taste. Mix the chicken into the marinade until saturated, and then set aside to marinate for 3 hours to 3 hours 30 minutes.
Preheat the tandoor oven to moderately hot. Thread the marinated chicken pieces onto skewers at least 3/4 inch apart. Coat the chicken pieces with the remaining oil, and keep the skewers on a tray to collect the drippings. Roast the chicken for 6 to 8 minutes, basting with the marinade at least twice. While cooking, make sure that the chicken pieces do not touch the sides of the tandoor.
Remove the chicken pieces from the skewers, cut all the pieces into halves, and serve.