Line a 9x13 pan with foil, spray very lightly with cooking spray.
If your chicken breasts are different thicknesses, pound them using a meat mallet until they are roughly the same thickness.
Pat chicken dry and place in pan.
Add sliced Swiss cheese on top of chicken breasts. You may have to divide a slice or two to cover the chicken completely.
In a bowl mix mayonnaise, sour cream, ½ cup Parmesan cheese, salt, pepper and garlic powder. Spread this over chicken. Sprinkle chicken breast with the remaining ¼ cup Parmesan cheese.
Bake for 30 mins. Take the chicken out of the oven, and insert a thermometer into its thickest part. The chicken is ready when it registers an internal temperature of 160 to 165 degrees Fahrenheit on an instant-read thermometer.
**Chicken will continue to cook after it's removed from the heat source and the internal temperature will rise about 5 to 10 degrees F. in the first few minutes it's off the heat.
Transfer the baked chicken breasts to a platter. Loosely tent with foil and allow to rest 5 minutes before slicing and serving.