Guinness glaze: Combine all the ingredients for glaze in a sauce pot and set the heat to medium. Whisk the mixture with a silicone whisk until combined. Keep slowly whisking until sugar is dissolved.
Let it simmer, stirring occasionally, for about 25-30 minutes. It will reduce in half. Raise the temperature to medium-high and boil for about 30 seconds. Stir and take it off heat. (Don't worry, it will thicken more as it cools.)
Sandwich: Grill the chicken and add a little glaze when it's almost done. Saute the chicken in glaze until fully cooked.
Build the sandwich and pour a little extra glaze between the chicken and cheese.