You've experienced both failure modes, so you already know the target: between them.
The shaping method: stack the butter pieces on butter paper, cover, and BEAT it with the rolling pin until it's one lump - beating makes it plastic without warming it the way kneading with hands would. Then fold the paper into an envelope of the right size (15×15cm for croissant dough, 12×12 for puff), and roll the butter inside it to fill every corner evenly. Uniform thickness edge to edge, that's the whole game.
The temperature rule: cold but PLIABLE. The block should bend without breaking and never feel soft. Fridge to hold it, out 2-5 minutes before locking in. Your shard experience = straight from a cold fridge, too brittle - it punches through dough. Your smear experience = sat out too long - it vanishes into the dough instead of layering.
Bend test before every lock-in: flexes like thick clay, go. Cracks, wait two minutes. Squishes, back to the fridge. And use 82% fat butter if you can get it - regular butter needs a properly cold room to behave.
