1Combine soy sauce, brown sugar, sesame oil, gochujang sauce, garlic and pepper in a large zip-top plastic bag. Place 2 tablespoons soy sauce mixture in a small bowl; set aside. Add chicken slices to remaining soy sauce mixture in bag; seal. Refrigerate 2 hours.
2Cook rice according to package directions.
3Remove chicken from bag; discard marinade. Heat a large skillet over medium-high heat. Add oil to pan; swirl to coat. Add chicken; cook 5 minutes on each side or until done. Sprinkle sesame seeds over chicken.
4To assemble lettuce cups: Place 3 tablespoons rice in each lettuce leaf; top each lettuce leaf with 1/3 cup chicken mixture, 2 cucumber slices, and about 1 1/2 teaspoons sliced green onions. Drizzle equal portions of the reserved soy sauce mixture over each lettuce cup. Serve immediately.