Preheat oven to 375℉.
Grease a baking sheet and set aside.
Pour panko breadcrumbs into a shallow dish.
Mix mayonnaise and Dijon mustard in a shallow dish.
Trim the chicken and place between 2 sheets of wax paper. Use a meat tenderizer to pound the chicken into a thin, even layer. Repeat this step for each chicken breast.
Salt and pepper the chicken to taste.
Place 2 slices of cheese and 3-4 slices of deli meat on each chicken breast. Fold over in half. Repeat for each chicken breast.
Dredge the chicken in the mayo and mustard mix. Be sure to hold it closed.
Coat each chicken breast with panko breadcrumbs. Secure with toothpicks and place on the baking sheet.
Melt 2 Tbs of butter in the microwave. Spoon the butter over the chicken breast.
Bake for 45 minutes or until internal temperature reaches 165℉.
While the chicken is baking, prepare the sauce. Mix together 6 tablespoons of mayo and 3 tablespoons of Dijon mustard.
After chicken is done, remove the toothpicks and serve.