Combine mustard, vinegar, salt and pepper in a large bowl. Add chicken breasts, tossing to coat well. Marinate for 2 hours.Blend or process crackers. Add cheese, butter and salt. Transfer mixture to a shallow soup plate.
Dredge chicken breasts in crumbs coating completely and pressing to help crumbs adhere.Transfer chicken to a baking sheet lined with parchment paper or aluminum foil.Bake in preheated oven 450°F (200°C) for 15 to 20 minutes until chicken is just cooked and golden.Serve.