Slice the calamari tubes into ¾-inch thick rings. Leave the tentacles whole.
In a medium bowl, stir 2 teaspoons kosher salt into 1 cup of buttermilk.
Add the calamari rings to the milk mixture and refrigerate for 30 minutes.
In a large bowl, whisk together the flour, cornstarch, baking powder, black pepper, and cayenne (make sure the mixture is well combined).
Using a pair of tongs or clean hands, take about half of the calamari rings, shake off the excess buttermilk, and place them in the flour mixture. Toss the calamari to coat evenly. Place the coated calamari on a wire rack. Repeat with the rest of the calamari.
Turn your oven on a low 150 degrees F. Prepare a large sheet pan and set a wire rack on top. Top the wire rack with paper towel.
Heat 4 inches of oil in a small cooking pot (I used a 2.5 quart pot) to somewhere between 350 and 365 degrees F. To test your oil, drop one calamari ring. Bubbles should immediately form around it and the calamari will begin to float.
Once your oil is ready, using a pair of tongs, gently add some of the calamari in and fry for about 3 minutes or until golden brown. Remove the calamari from the oil with a slotted spoon and arrange it on the paper towel on top of your prepared wire rack. Immediately season with kosher salt, then transfer to your pre-heated oven to keep warm while you work on the rest. Repeat until you have fried all the calamari.
Once finished, add all of fried squid to the hot pan with the garlic butter and sliced peppers, sprinkle with chopped parsley, and quickly and gently give it a toss. Serve immediately with extra sliced peppers on the side.