Begin by cleaning the mushrooms with a damp cloth and slice each mushroom into about ½-inch thick pieces.

In a medium bowl, whisk together flour, cornstarch, garlic powder, onion powder, paprika, salt, black pepper, and optional cayenne pepper.

Pour buttermilk into a shallow dish and dip each mushroom slice thoroughly into the buttermilk.

Transfer the dipped mushroom slices into the seasoned flour mixture and toss gently to coat.

Add vegetable oil to a large skillet and heat over medium-high heat until it reaches 350°F (175°C).

Carefully place coated mushroom slices in the hot oil and fry in small batches for about 3-4 minutes, flipping halfway through.

Use a slotted spoon to remove the fried mushrooms and place them on a plate lined with paper towels.

Arrange the cooked mushrooms on a serving platter and serve hot with ranch dressing.