Melt butter in a skillet on top of the stove over med-high heat. Combine garlic powder, 1 teaspoon of Cajun seasoning, and a pinch of salt in a bowl. Toss in shrimp and stir to coat. Cook in melted butter for 3 to 4 minutes turning once. Shrimp should be pink when cooked. Set in refrigerator to cool.
Mix eggs, onion, celery, mayonnaise, mustard, pepper, salt, and the other 2 teaspoons of cajun seasoning together. Add cold shrimp and combine with the egg mixture. Serve on hoagie buns or with crackers.
Notes
You can add less cajun seasoning to make this dish less spicy. You can also chop up the cold shrimp before adding it to the egg mixture.