Pat the 1 pound button mushrooms dry with paper towels (if not already dry) and ensure stems are trimmed.

In a medium bowl, whisk together 1 cup all-purpose flour, 1 cup buttermilk, and 1 large egg until smooth and lump-free.

In a separate shallow bowl, combine 1 cup panko breadcrumbs with 1/2 teaspoon salt and 1/4 teaspoon black pepper; stir to distribute evenly.

Heat vegetable oil in a deep-fryer or a large, heavy pot to 375°F (use enough oil to submerge the mushrooms).

Working in batches to avoid overcrowding, dip each mushroom into the batter, allowing excess to drip off, then roll the battered mushroom in the panko mixture and press lightly so the breadcrumbs adhere. Place coated mushrooms on a plate or tray while you finish coating the rest.

Fry the mushrooms in the hot oil in batches until golden brown, about 3–5 minutes per batch, turning as needed for even color. Do not overcrowd the fryer or pot.

Transfer fried mushrooms to paper towels to drain. Repeat until all mushrooms are fried, then serve immediately.