Cut the chicken breast in halves or thirds if they are big. Sprinkle salt and pepper on both sides.
Heat the skillet over medium high heat and add butter or oil.
When the skillet is hot, seared the chicken breast for about 3-4 minutes per side, depending on how thin you cut them. Once you have turned them, on the last 2 minutes of cooking time sprinkle some brown sugar on each piece and continue cooking. The brown sugar will began to melt on the chicken.
Once they are cooked remove them from the pan. Add the brie cheese immediately.
Cut in half and toast the buns with a little butter on a skillet.
You can now start assembling your sandwich. I like to start with the mayo, add the chicken, tomato, red onion, and watercress.