Keep the butter at 15°C, do three letter folds, and you get 27 butter layers, at least in theory.
So what happens once the butter goes past 20°C?

This is an overengineered croissant report that runs from mixing the dough all the way to slicing the finished crumb.

In this video:
- Low-speed mixing keeps gluten minimal so the butter layers won't tear
- The dough rests 12 hours at 4°C so the gluten can relax
- 84% butterfat butter tuned to 15°C: firm, yet bendable
- 3 letter folds: 3 → 9 → 27 butter layers in theory
- Above 20°C, butter softens, smears into the dough, and the layers collapse
- Sheeted to 3.5mm, each butter layer is roughly 0.04mm
- Triangles cut 90mm at the base and 250mm tall, then rolled into 7 spiral ridges
- Proofed at 26.5°C and 75% humidity until 2.5x in size
- At 200°C, water in the butter flashes to steam and pushes the layers up like pistons
- Cooled and sliced to show the honeycomb crumb