Combine sauce ingredients and set aside.
Trim legs and feelers off prawns with scissors. If you are using prawns with heads on, trim off part of the head just behind the eyes. With sharp knife, score backs of prawns through shell and remove vein. Rinse with cold water and pat dry.
Heat wok or skillet over high heat, add oil and heat until oil starts to smoke lightly. Add garlic and pepper flakes and brown lightly. Add prawns and cook for about 2 minutes, tossing to cook evenly.
Pour in sauce mixture. Stir well and cook until prawns turn pink and completely opaque, about 3 minutes. Serve immediately. Reserve shells for Shrimp Stock.