Use a clean dish towel to wipe the mushrooms clean, remove stems, and discard.
In a bowl, mix together flour, cornstarch, baking powder, salt, garlic powder, and black pepper. Add in the water to create the batter.
Dip each mushroom in the batter, allowing the excess to drip off.
Coat each mushroom in Panko bread crumbs.
Heat about 2 inches of oil in a large stock pot to about 350 degrees Fahrenheit. Carefully add the mushrooms to the hot oil using tongs. Allow the mushrooms to cook for about 7-10 minutes, or until they are golden brown and crispy.
Carefully remove mushrooms from the oil to a paper towel-lined plate. Allow the mushrooms to cool for a few minutes before serving with ranch dressing.