Katherine, thanks for your reply. I would have watched the videos if I had known they were there. I did watch the two videos that the formula mentions, both about crushing the peppermints. The CL website video page has links to plenty of videos, but none about the 2023 holiday cookie. There’s probably an email that mentions them that I haven’t read or have forgotten.
I don’t think the knife is the issue — it’s a stainless-steel chef’s knife with a 7-5/8″ blade. Longer would be better, but it should be adequate. I see from watching the videos that your technique was a little different from mine: you laid the entire edge along the surface of the shortbread, while I tended to start nearer the tip, in the curved part of the edge, and then press down, rotating the rest of the blade into the shortbread — more slicing than chopping. I think the longer and straighter the edge meeting the shortbread, the less chance of off-course fractures (my fractures went about 120 degrees from the cut line).
Beyond that, I probably overmixed the dough. I rarely make crusts or shortbread, and I couldn’t seem to get it right with either the pastry blender or the food processor. It was certainly finer-grained than the dough that went into the pan in your video.