Beat the chicken fillet with a rolling pin and cut into strips about 2.5 cm wide. In a large plastic bag, combine milk, garlic, brown sugar and spices. Put the chicken fillet in this bag with the marinade, shake the bag well so that the meat is completely in the marinade. Put the bag in the refrigerator for 8 hours, or better overnight.
In a bowl, combine all the sauce ingredients. Cover the bowl with cling film and refrigerate until serving. Remove the chicken fillet from the marinade and skewer.
Heat the grill to medium temperature and place the chicken on the wire rack. Fry the chicken skewers for about 2-3 minutes on each side.