bowlturner's answer is correct, and will probably give you the results you are looking for. But it inspired me to experiment, mostly to see how sharp an edge I could get.
As you can see, just setting down a piece of hardwood makes an obvious, if not sharp, line. The main problem is getting into the corner. I tried to only scorch the pine, so I couldn't wait for the flame to burn the inside edge.
This time I slowed down and concentrated on getting the flame into the corner. I also used a steel guide, but I didn't see a difference from the hardwood guide.
As you can see, this one is quite well done (burnt), but it does have a sharper edge.
Not great, but not as bad as I thought. The heat melts off the adhesive and it's easy to burn through the tape a little bit, but it made an okay line. I cleaned off the adhesive with mineral spirits.
I also tried routing out a shallow area to receive the burning, with the intention of planing off the high spot. This didn't work though, because it creates another inside edge that the flame can't reach. But if you could somehow get the burned part right up next to a freshly-planed edge, I'm confident that this would give you the cleanest, sharpest edge possible.
Source for all images: me