Slice 12 mini Shanghai bok choy in half lengthwise. Wash to remove any grit. Plunge into a large pot of boiling water for 30 sec. Drain and rinse until cold. Pat dry. Whisk 4 tsp soy sauce with 1 tbsp rice vinegar, 2 tsp sesame oil and 1 tsp sugar in a large bowl. Whisk in 2 tbsp vegetable oil. Add bok choy and toss until coated.