In a small mixing bowl combine together the mayonnaise, garlic powder, and Parmesan cheese. Then sit aside.
Next, apply butter to one side of each piece of sourdough bread and place one piece, butter side down, onto a heated panini press.
Now you will layer on the other ingredients! First, lay down a piece of Swiss cheese onto the bread already on the panini press, followed by approximately 3-4 pieces of ham, the tomato, add your other piece of cheese, and finally before you add the final piece of bread, put a good amount of the sauce you made earlier onto the sandwich and apply the send piece of bread, butter side up!
Close your panini press and wait for approx 2-3 minutes before lifting the lid to check for desired doneness. Once finished, cut the sandwich in half and admire your creation before digging in! Enjoy!