Start by combining all the dry spices in a small bowl, whisking them together until evenly distributed. I always make sure to break up any clumps with the back of a spoon for even coating. This seasoning blend can be made up to a week ahead and stored in an airtight container.
Pat the peeled and deveined shrimp completely dry with paper towels – this step is crucial for achieving that signature blackened crust. Toss the dried shrimp with the seasoning blend, making sure each piece is evenly coated. Work the seasoning in with your hands to ensure every curve and crevice is covered.
Place your cast iron skillet over medium-high heat and let it preheat for 3-4 minutes until it's smoking hot. Add the avocado oil and swirl it around to coat the bottom of the pan. The oil should shimmer and move freely when the pan is properly heated.
Add the seasoned shrimp to the hot skillet in a single layer, working in batches if necessary to avoid overcrowding. Let them cook undisturbed for 1-2 minutes until the bottom develops a dark, crusty sear. Flip each shrimp using tongs and cook for another 1-2 minutes until they're just opaque and cooked through.
In the final minute of cooking, add the butter and minced garlic to the pan, stirring gently to coat all the shrimp. The butter should foam and turn golden, creating an aromatic garlic butter sauce. Remove from heat immediately to prevent overcooking, as shrimp continue cooking from residual heat.