First, mix together the batter ingredients: potato starch, baking powder, egg yolk, salt, and pepper. Then add enough water until you have a thick batter.

Then, dip mushrooms into batter until they are covered. Transfer them to a large pot or deep fryer, heat a few inches of oil to 350 degrees F. Fry mushrooms until golden brown. Blot on paper towels and sprinkle with more salt immediately. Serve hot!