Mix cold water and kosher salt together in a large bowl until salt dissolves. Add chicken legs. Cover entire bowl with plastic wrap and refrigerate for 2 hours.
Remove bowl from the refrigerator; drain and pat chicken dry with paper towels.
Place chicken legs in a plastic bag with 1 cup all-purpose flour and shake to coat.
Pour buttermilk into a bowl. Mix remaining flour, baking powder, salt, and cayenne in another bowl. Dip each chicken leg in buttermilk; move to flour mixture and coat well.
Heat canola oil in a deep-fryer to 350 degrees F (175 degrees C).
Fry chicken legs in hot oil in batches until no longer pink at the bone and the juices run clear, about 15 minutes. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C).