Wipe the king oyster mushrooms clean, trim any dry ends and tear them lengthways into irregular strips approximately 1.5–2 cm thick.
Combine the plain flour, cornflour and all the dry coating seasonings in a wide, shallow bowl. Mix thoroughly.
In a separate bowl, whisk together the flour, cornflour, paprika, garlic powder, onion powder and salt. Gradually add 250 ml of cold sparkling water, whisking until smooth. Add the remaining water as needed to make a thick but pourable batter that coats the mushroom strips.
Dip each mushroom strip into the wet batter, allowing excess to drip off, then press it into the seasoned flour. Dip it into the wet batter again, followed by a second coating of seasoned flour. Press gently to create an uneven, textured surface.
Pour the frying oil into a deep, heavy-bottomed saucepan, ensuring the pan is no more than one-third full. Heat to 170–180°C (340–355°F).
Carefully add a few coated mushroom strips to the hot oil, without overcrowding the pan. Fry for approximately 5–7 minutes, turning occasionally, until golden brown and crisp. Maintain the oil temperature throughout frying.
Transfer the fried mushrooms to a wire rack set over a baking tray. Allow excess oil to drain, then serve immediately with your preferred dipping sauce.