Preheat your oven to 400°F (about 200°C).
In a shallow bowl, whisk together the egg, baking powder, all-purpose flour, cornstarch, pepper, soda water, and salt until smooth.
Dip each mushroom in the batter and let any excess drip off. Place on a baking sheet lined with parchment paper.
Bake for 15 minutes or until the mushrooms are golden brown and crisp.
Serve with your favorite dipping sauce, such as soy sauce, sweet chili sauce, or even ranch dressing.