Preheat oven to 375°. Place 4 (8-ounce) skinless, boneless chicken breasts between 2 sheets of heavy-duty plastic wrap; gently flatten each piece to 1/3-inch thickness using a meat mallet or heavy small skillet. Place chicken into a large bowl. Pour 1 cup low-fat buttermilk (1%) over chicken; cover and marinate in the refrigerator for 1 hour. Place 1 cup toasted pecans and 1 cup panko (Japanese breadcrumbs) in a food processor; pulse until fine. Transfer mixture to a large rimmed dish. Remove the marinated chicken breasts from buttermilk; shake off excess buttermilk. Evenly coat chicken with breading mixture. Place chicken onto a baking sheet coated with cooking spray; bake 25–30 minutes or until crispy and golden. Season with salt and pepper; serve immediately.