Combine all ingredients in a large saucepot and bring to a boil to dissolve the salt.
Remove from heat and let cool slightly.
Place a spatchcocked chicken breast side down in a 9x13 casserole pan and pour the brine overtop.
Cover with a press and seal wrap and let completely cool, then place in fridge until needed.
Notes
I pierced the meat of the chicken, under the skin - not through the skin, with a long toothpick to assist the brine soaking further into the muscle tissue.