We use Sheppard Brackets brand floating mounts. Floating shelf Bracket- 3/4" Backplate- for shelves 1 1/8"-1 1/2" thick 

Recessed mount, we router out a pocket to fit the bracket flush. This would be needed if you plan to install your mantle to your finished wall (such as finished shiplap siding.) 

Non-recessed, we do not router out the pocket for the bracket. This will leave the bracket sit proud about 1/8". This is an option if you plan to wrap around your mantle (such as stone veneer.)