Dissolve the kosher salt and granulated sugar in one quart of cold water. Place chicken in a ziptop bag and pour cold water over. Seal bag and place in refrigerator overnight to brine.
For the Dredging
Combine paprika, black pepper, cayenne pepper, and MSG (if using) in a small bowl and set aside.
Whisk together milk and eggs in a shallow bowl and set aside.
Whisk together flour, nonfat dry milk powder, baking powder, ½ tsp kosher salt, 2 tsp granulated sugar, and 2 tablespoons of the spice mixture in a large shallow bowl to combine. Drizzle 3 tablespoons of the milk/egg mixture into the flour mixture and rub with your fingertips until the flour is like wet sand.
Frying and Assembling
In a large Dutch oven, heat the peanut oil to 350° F.
Remove the chicken breasts from the brine and pat them dry with paper towels. Season on all sides with the remaining spice mixture. Transfer the chicken to the milk mixture and coat fully. Allow excess milk mixture to drip off and then transfer to the flour mixture, Press down firmly to adhere as much of the flour mixture to the chicken as possible. Shake off the excess flour and lower the chicken into the hot oil. Repeat with the rest of the chicken (cutlets or nuggets), frying until golden brown on all sides (about 4 minutes for whole pieces and about 2 minutes for nuggets).
Transfer chicken to wire racks set over baking sheets to drain.
If making sandwiches, cut gluten free hamburger buns in half horizontally and spread each side with butter. Toast in pan until lightly browned, or wrap in foil and place in a 350° F oven for a few minutes, just to soften buns slightly.
Place chicken cutlets on toasted buns and add pickle slices and mayonnaise, if desired. Serve warm.
*Chick-Fil-A's chicken is pretty salty, but if you'd like a little less salty chicken, reduce the kosher salt in the brine by half.