From here I have two options for how to style the back build. I chose to face the front of the frame on the side for this tutorial so you can see the burnt wood style along the side of the frame.

You could also stack the frame so they are back to back. This would mean that the burnt wood is facing the back and the sides match with more of a wood finish and less texture on the side. If I did it that way, the frame height is 1.5″ so I would just need to trim 1″ off that way.

Since the frame moulding is 2.5″ in width, I trimmed off 0.5″ from the rabbet side and kept the flat side so in the end I had the 2″ thick back build I wanted with the “front” of the frame now sideways to have the charcoal type look along the side of the frame.