Add the chicken broth, beef broth, beef bouillon, onion and garlic powder, and Worcestershire sauce to a medium saucepan. Stir to combine.
If a darker color is desired, add 2-3 drops of Kitchen Bouquet browning and seasoning sauce.
Combine the cold water and corn starch in a small Tupperware with a lid. Shake to combine.
Bring gravy to a boil. Whisk in cornstarch mixture. Decrease to medium low. Continue to whisk to ensure that it blends in smoothly.
Let it gently simmer until its of desired thickness. The longer it simmers, the more concentrated the flavor will become.
Remove from heat. Swirl in one tablespoon COLD butter just before serving. This adds a silky, velvety finish.