Cut 2" square cube from top of wood post for 2"x2" wood rail?
You could rough it out with a drill, maybe a router. Then tidy up the corners and edges with a chisel and mallet.

You could use a saw to establish the edges, then finish off with chisel and mallet.

Given that people such as Paul Sellers routinely cut blind mortices remarkably quickly using only a chisel and hammer, this could certainly be done that way too.