[Steve Neul]

For that application I think two rabbeted pieces would work better. You don't really have enough vertical height of the mantle for a french cleat to work.

 

[Steve Neul]

What would be better is to chisel out a few bricks and mortar in some wooden blocks sticking out from the wall to slip the mantle over. If the mantle had been planned for when the fireplace was being built this is a common way to do it.