4.

Combine cornstarch and flour in a large bowl and stir with chopsticks until roughly combined. Add soda and stir very rapidly with chopsticks while shaking bowl. Do not overmix—there should be plenty of lumps and bubbles. Add sliced mushrooms to bowl and turn to coat. Working one piece at a time, lift mushroom, allow excess batter to drip off, then slowly lower into hot oil. Repeat until all mushrooms are in the oil. Cook mushrooms, agitating oil and turning mushrooms occasionally with a wire mesh spider or long chopsticks until pale golden brown and completely crisp, about 1 1/2 minutes. Transfer to a paper towel-lined plate and season with salt immediately.