Pat each chicken breast dry using paper towels, then with a sharp knife, slice them horizontally to form two thinner breast halves. Place each piece between two sheets of plastic wrap and gently pound them with a meat mallet or rolling pin until evenly flattened, taking care to avoid tearing the meat. Lay a slice of ham and a portion of shredded or sliced Swiss cheese atop each flattened piece. Roll the chicken tightly around the filling, tucking in the sides slightly, and place each roll onto a new sheet of plastic wrap. Wrap each bundle securely, twisting the ends like a tootsie roll to form firm rolls, then refrigerate for at least 30 minutes or up to one day. Preheat the oven to 400°F. Unwrap the chicken rolls, season them with salt and pepper, and set aside. Pulverize the corn flakes in a food processor to form fine crumbs, and pour them into a shallow dish. Melt the butter separately in another shallow dish. Dip each chicken roll first into the melted butter, then coat thoroughly with the cornflake crumbs, pressing gently to adhere. Arrange the coated rolls onto a lightly greased baking sheet. Bake on the center rack for approximately 30 minutes, or until the internal temperature of the chicken reaches 160°F, ensuring the thermometer measures the chicken itself, not the filling. Prepare the sauce by combining the mayonnaise and mustard in a bowl, adjusting the mustard to taste. Serve alongside the baked chicken.