Excellent Tikka-Masala spice mix is available ready-made in the shop. You can make a homemade spice mix like this (from ground spices): 2 tsp cheese cumin, 2 tsp coriander, 2 tsp paprika powder, 1 tsp turmeric, 1 tsp cayenne pepper (less if you want it milder), 1 tsp garam masala, 1 tsp garlic powder, 1 tsp ginger powder, 1/4 tsp cloves.
Cut the chicken into fair sized pieces and marinate with the yoghurt and tikka-masala seasoning mix in the fridge for at least a 2 hours.
Place the chickens loosely on skewers and place in a 270C oven for 11 minutes. An oven with grill racks is even better. Grill the chicken skewers quickly in the hot oven. Turn over occasionally. The surface of the pieces may even get a little charred.
Meanwhile, put the sauce ingredients in a saucepan and start cooking, about 20 minutes.
Finally, add the grilled chicken pieces to the tikka masala sauce and simmer for 5 minutes. Check the taste and enjoy.