Rinse prawns, pat completely dry and marinate in cornstarch and 1/2 tsp kosher salt for 5 minutes. Heat your wok or large skillet over high heat. When wok is hot, add 2 tbl cooking oil. When oil is hot and just starting to smoke, add prawns. Fry until they are half-done, approximately 1-2 minutes. Remove from wok, leaving the oil in the wok.
Turn heat to medium-high. Add the garlic, fry for 10 seconds. Add the brandy, salt, sugar, butter. Cook sauce for 1 minute to thicken slightly. Add the half-cooked prawns and green onion. Fry until prawns are cooked through, about 2 minutes (depends on size of your prawns)