Butter browned Panko breadcrumbs: Add 3 tablespoons of butter to a skillet over medium heat. Once the butter is melted and bubbly add 3/4 cup of Panko breadcrumbs. 

Cook the breadcrumbs, stirring often until they start to look golden brown like the exterior of fried chicken. Once they are browned remove them to a paper towel-lined plate to cool.

Panko chicken: preheat your oven to 450 F / 230 C. 

Butterfly your chicken breast and then cut it into two pieces. Butterflying cuts the chicken breast parallel with your cutting board so that you will have a flat, thinner piece of chicken. Here's a short butterflying tutorial if you've never butterflied a chicken breast before. After you've butterflied the chicken, go ahead and cut the large chicken piece in half so that you have two thin cutlets. 

Cover the cutlets with plastic wrap or even better, a large zip-top bag that you have laid on top of the chicken. Using a meat mallet or a heavy pot, pound the chicken through the plastic to attempt to flatten each piece of chicken until it is less than 1/4-inch thick. This might take some effort and it will expand the width of your chicken piece considerably. After the chicken has all been pounded, cut each cutlet into pieces that are around the size and shape of your bread. The goal is to get 2 hand-sized, quarter-inch thick cutlets out of one chicken breast. 

Set up your chicken dredging station. You will need two bowls or large plates for this. 

The first bowl will contain 1 whole egg, 1 tablespoon of mayonnaise, and 1 teaspoon of Dijon mustard. Whisk to combine well. The second bowl will have the butter-browned Panko breadcrumbs.

Line a sheet pan with aluminum foil. Spray the foil with cooking spray, this is where your chicken will go when you have coated it in breadcrumbs.

For each chicken cutlet, dip into the egg mixture (ensuring that it's fully coated everywhere) and finally press into the Panko breadcrumbs until each piece is fully coated. Place on the cooking spray sprayed aluminum foil. 

Bake for 6 minutes, remove from oven, and flip each chicken cutlet. Bake for an additional 5 minutes. 

Once the chicken is done, remove the sheet pan from the oven and turn the oven on to broil. 

Add slices of ham and cheese on top of both pieces of chicken. Place the sheet pan back into the oven and broil to melt the cheese. You can toast your bun at this point as well if you want. You will want to keep an eye on the cheese to make sure it doesn't burn. It should probably take 3 to 4 minutes to get all melty. 

Dijonnaise: while the cheese is melting, combine mayonnaise and Dijon mustard in a small bowl and stir to combine well. 

Store in a sealed container in the refrigerator for up to a week. 

Sandwich assembly: spread some Dijonnaise on the bottom of the bun. Top it with the chicken, ham, and melty cheese, and then close it all with the top bun. 

Serve and enjoy.