Mix flour, cornstarch, baking powder and salt in a bowl.
Add water to make a batter.
Dip clean mushroom into batter letting excess batter drip off.
I find it easier to stick a toothpick into the mushroom to dip it.
After dipping into batter roll in Panko breadcrumbs to coat.
Deep fry in oil until golden.
We like to serve with cocktail sauce.