Bring large saucepan filled half way with water to a boil, add chicken breast and cook on medium heat until it is cooked thru and chicken is no longer pink inside. Drain the water and let it cool. Using hands or two forks tear the chicken into small pieces.
Warm tortillas on top of the stove for few seconds, it will make them soft and flexible.
In a mixing bowl combine chicken, ground cumin, cheese, chillies, Mexican White Sauce, salt and pepper. Give it a good mix.
Place 1/2 cup of the chicken mixture on center of each tortilla. Fold the sides, then the bottom side up, to cover the filling, roll over to create chimichangas.
Brush each chimichangas with butter and bake for 15-20 minutes or until golden color. Serve with leftover Mexican White Sauce, Guacamole or cabbage salad.