Prepare the Korean sauce by combining the soy sauce, brown sugar, fresh garlic, ginger, sesame oil, garlic powder, onion powder, and red pepper flakes in a small bowl. Whisk until combined. In a separate bowl, mix the cornstarch and water until it becomes a thin paste. Then stir it into the Korean sauce.
In a large skillet, brown the ground beef and onion over medium high heat, breaking the meat up with the spoon as it cooks, until no longer pink. Stir in the water chestnuts.
Add the Korean sauce to the beef and bring to a simmer. Cook about 5 more minutes or until the sauce begins to thicken.
To assemble the lettuce wraps
Core lettuce and gently remove leaves to avoid tearing. Rinse and dry.
Place a spoonful of the Korean Beef in each lettuce leaf. Top with shredded carrots and green onions. Drizzle with your choice of spicy mayo or yum yum sauce if desired. Serve immediately.