Place shrimp into a resealable container or baggie.
In a large bowl whisk together the buttermilk and mustard.
Pour mixture in with the shrimp, seal, and shake gently to coat.
Refrigerate for 1 hour.
Whisk together the fish fry mix and cayenne.
Heat 3" of oil in a Dutch oven to 325 F.
Working in batches, remove shrimp from container. Shake off excess marinade then transfer to the fry mix. Roll and coat. Shake off an excess coating and place in hot oil.
Fry shrimp 1 1/2 minutes per side then transfer to a wire rack or paper towel-lined plate to drain and cool.
Serve with your favorite dipping sauce or on a po boy.