What are these walls?
This is almost certainly an attempt to shield the wall from woodstove heat. It is most definitely not to code. It could be made much safer by adding a metal shield 1-2" spaced from the wall anywhere the wall is less than 36" from the stove. If the outlet has to be exactly there, then good luck cutting thru what might be a stud. If it is not a stud, then mark your hole, cut it, then install a metal shield before lighting your stove again. The outlet itself can't stand the heat from a woodstove, either, so it must be behind the shield also. Spacing the shield farther from the wall is not a problem, tho it may not look like you want it to look. Sometimes moving the outlet a few inches one way or the other makes a big difference on how difficult the project is.