Remove chicken pieces from buttermilk one at a time and coat each piece all over in Grape-Nuts cereal mixture, shaking off excess. Place chicken side-by-side in prepared pan; discard buttermilk. Bake chicken for 4-6 minutes per side for tenders, or 10–12 minutes for breasts, depending on the thickness, until they are crispy and cooked through. Serve with yogurt dipping sauce: stir together all dip ingredients until blended. Serves 4-6.