Add all brine ingredients to a saucepan and boil for 10 minutes to allow sugar and salt to fully dissolve.
Place brine in the fridge to cool. - These first steps can be done ahead.
Cut out the backbone of each Cornish hen so they are able to lie flat.
Place thawed Cornish hens in a gallon storage bag and pour in brine and ensure birds are fully submerged.
Let the Cornish hens brine for 2 hours.
Remove from bag, rinse under cold water and pat dry.
Place hens on grate over hot charcoal, breast side up and let cook till the bottom begins to blacken. The time will vary depending on the heat of your coals.
Flip so the breast side is down and let cook till the skin crisps and begin to blacken.
Use a meat thermometer to check for doneness. If the meat has not reached 165 degrees move the birds away from direct heat so they can continue cooking without burning.
Let rest 5 minutes and serve along with a dollop of wasabi.