01 - In a large bowl, whisk buttermilk with hot sauce if using. Add mushrooms and toss to coat thoroughly. Let marinate for 15 minutes to absorb flavors.
02 - Combine flour, cornstarch, garlic powder, onion powder, smoked paprika, cayenne, salt, and black pepper in a shallow dish. Mix well to distribute spices evenly.
03 - Remove mushrooms from buttermilk, allowing excess to drip off. Press into flour mixture to coat thoroughly. For extra crunch, dip dredged mushrooms back into buttermilk and repeat the flour coating process.
04 - Heat oil in a deep skillet or pot to 350°F. Fry mushrooms in batches for 4 to 5 minutes, turning occasionally, until golden brown and crispy. Drain on paper towel-lined plate to remove excess oil.
05 - Melt butter in a saucepan over medium heat. Whisk in flour and cook for 1 to 2 minutes until lightly golden. Gradually whisk in milk, then add black pepper, salt, and garlic powder. Cook, whisking constantly, for 3 to 5 minutes until thickened and smooth.
06 - Arrange fried mushrooms on a serving platter. Pour warm gravy over the top or serve on the side for dipping. Best enjoyed immediately while crispy.