Place frozen shrimp in a bowl of cold water and allow to thaw for 20 minutes or so. Drain water from the bowl then run cold water over the shrimp to get rid of any ice crystals that may have formed.
Use a paring knife or steak knife to gently cut a slit lengthwise on the underbelly and the backside of the shrimp to remove the digestive tract. Discard the digestive tract. Rinse the shrimp really well.
Place the shrimp in a single layer on the prepared baking sheet to absorb all of the moisture from the shrimp. Use a couple of additional paper towels to pat dry the surface of the shrimp.
Place the shrimp in a large bowl and season with kosher salt, black pepper and Cajun seasoning and mix until all shrimp are coated.
Add grape-seed oil or olive oil and butter to a skillet and place over medium high heat. Swirl the cooking oil around the pan. Place the shrimp in a single in the skillet and sear for 2 to 3 minutes. Flip the shrimp and sear for another 2 minutes. Turn off the heat.
Remove shrimp from skillet and transfer to a serving bowl or platter. Do not keep the shrimp in the skillet as the residual heat will continue to cook the shrimp which can result in them becoming overcooked and rubbery.
Always de-vein shrimp even when the packaging indicates already de-veined it doesn't hurt to go through the shrimp to make sure they're clean
For perfectly seared shrimp, make sure they're completely dry before adding them to the skillet
Frozen raw shrimp are the freshest and should be used instead of defrosted shrimp at the seafood counter at your local grocery store
Cook the shrimp in batches to prevent the shrimp from steaming instead of searing