Are you talking a square or round mortise? if round, use a drill press and Forstner bit. If square, then I'm not quite sure what you're talking about. My guess is that you could use the Forstner to clear most of it and then carefully chisel it square. If you're not comfortable using a chisel, do a trial run on some scrap of the same material. Sharpen your chisels before you start--razor sharp. This may not work well if the corners have complex beads and coves. Picture please.
The more I do, the less I accomplish.