01 - In a large bowl, whisk buttermilk with hot sauce (if using). Add mushrooms and toss to coat. Let marinate for 15 minutes.
02 - In a shallow dish, combine flour, cornstarch, garlic powder, onion powder, smoked paprika, cayenne, salt, and black pepper.
03 - Remove mushrooms from buttermilk, letting excess drip off. Dredge in flour mixture, pressing to adhere. For extra crispiness, dip dredged mushrooms back in buttermilk and then again in the flour mixture.
04 - Heat oil in a deep skillet or pot to 350°F. Fry mushrooms in batches for 4–5 minutes, turning occasionally, until golden and crisp. Drain on a paper towel–lined plate.
05 - In a saucepan, melt butter over medium heat. Whisk in flour and cook for 1–2 minutes until lightly golden. Slowly whisk in milk, then add black pepper, salt, and garlic powder. Cook, whisking constantly, until thickened (about 3–5 minutes).
06 - Arrange fried mushrooms on a platter and pour warm gravy over them, or serve gravy on the side for dipping.