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Adjust oven rack to middle position and heat oven to 350 F. Whisk melted butter, mustard, vinegar, garlic, rosemary, ½ tsp salt, ½ tsp pepper and cayenne in bowl until combined. Stir in panko until fully combined.
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Spread panko mixture in even layer on rimmed baking sheet, breaking up any clumps. Bake, stirring every 5 minutes, until golden brown, 15 to 18 minutes. Transfer crumbs to 13x9 baking dish and let cool completely, about 10 minutes. Break up any large crumbs with your fingers.
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Increase oven temperature to 400 F. Set wire rack in now empty baking sheet. Place flour in shallow dish. Whisk egg white together in second shallow dish. Stir Parmesan into cooled crumb mixture. Pat tenderloins dry with paper towels and season with salt and pepper.
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Working with one tenderloin at a time, dredge in flour, shaking off excess; dip in egg whites to thoroughly coat, letting excess drip back into dish; then coat with crumbs, pressing gently to adhere. Transfer tenderloins to prepared rack. Roast until pork registers 145 F, 25 to 30 minutes. Transfer to carving board and let rest 10 minutes. Slice into ½ inch thick slices.