Dissolve salt in 2 cups of warm water, place Cornish game hens in large pot, then fill the rest of your brining receptacle with cold water.
Brine thawed game hens in your refrigerator and soak grilling planks overnight.
Remove game hens from brine and pat dry. Stuff game hens with shallots, garlic, pearl onions, and herbs, then rub the skin with butter, salt, and pepper.
Place stuffed game hens on soaked grilling planks. Place grilling planks on hot grill. Let the grilling plank begin to smoke (approximately 10 minutes), reduce heat to low, or move grilling plank to indirect heat area for slight smolder.
Allow game hens to cook for 40-50 minutes. Check regularly to prevent cooking too quickly.
Use meat thermometer to test for doneness in the thickest part of the chicken. The internal temperature should be 165°F.
Carefully remove your game hens from the grilling planks, cover with foil and allow to rest for 5 to 10 minutes.