Step One: Cook white rice according to package directions, omitting any salt, until the water is just absorbed. Set aside.
Step Two: Place chicken breasts in a large stockpot and cover with 2 quarts water. Add in bouillon cubes. Bring to a boil, then cook over medium heat until the chicken is cooked through, about 20-25 minutes. Remove the chicken from the pot and allow to cool a bit. Roughly chop chicken and set aside.
Step Three: Melt 4 tablespoons butter in a skillet. Stir in flour to make a roux. Continue cooking for a minute or so, until heated. Add the roux to the broth and stir until well combined. Rinse out the skillet, and then melt the remaining 2 tablespoons of butter. Add the onion, green pepper, celery and pimentos to the skillet. Cook until slightly tender. Add in the chopped chicken pieces and stir until well combined. Stir in cooked rice to taste.