Combine coffee, lager, sugar, lime juice, chipotles, adobo sauce, salt, and pepper. Reserve half of the marinade for future use, and use the remainder to marinade the chicken for 4 hours.
Heat grill on high. Pat chicken dry and grill for 6-7 minutes a side until 165 deg F.
As chicken grills, set small saucepot on medium-low heat (great time to use the side burner on your grill). Whisk in the loosened cornstarch and cook until just thickened.
Once chicken is cooked, let rest for 3 minutes until slicing, topped with the sauce. Served paired with grilled peppers and onions as shown.