Cut chicken breasts in half horizontally so you have 2 thinner fillets. Cut each piece into squares or rectangles that are 1-2” long on each side.
Whisk pickle juice, milk, brown sugar, salt, onion powder, black pepper, and smoked paprika together in a mixing bowl.Pour cut chicken into the bowl and toss so its fully covered by the marinade.
Cover the bowl and refrigerate for at least half an hour and up to 4 hours.
Heat a grill pan or cast iron skillet over medium heat and lightly spray with oil.
Place chicken on the pan in an even layer. Cook for 4 minutes without moving so you get those nice brown grill marks, flip and cook for another 4 minutes. Check for an internal temperature of at least 165F and remove from heat. Serve warm.