In a small bowl, combine the panko breadcrumbs, Old Bay seasoning, marjoram, garlic powder, and kosher salt. Add the buttermilk to another small bowl.

Wash and dry the mushrooms thoroughly. Add the mushrooms to a zipper storage bag, add the flour, and shake to coat.

Dip the coated mushrooms in the buttermilk and roll them in the seasoned panko breadcrumbs.

Heat 1 1/4 inches of oil in a Dutch oven, heavy pot, or fryer to 375 degrees.

Carefully add a few breaded mushrooms at a time, frying until golden brown, about 2-3 minutes. Remove to paper towels to drain.