Clean the mushrooms thoroughly, removing stems if necessary.
In a medium bowl, whisk together flour, cornstarch, garlic powder, onion powder, salt, and pepper.
In a separate bowl, whisk the egg and buttermilk together.
Dip each mushroom in the wet mixture, then coat in the flour mixture. Repeat for extra crispiness.
Roll the coated mushrooms in breadcrumbs until evenly covered.
Heat vegetable oil in a deep fryer or large pot over medium-high heat (around 350°F/175°C).
Fry the mushroom bites in batches for 3-4 minutes or until golden brown. Remove and drain on paper towels.
Serve hot with your favorite dipping sauce. Enjoy!