Substitutions for "European style" butter in croissants
A mix of clarified and ordinary unsalted butter works well.

I used clarified butter that was simmered for a long time to be sure the water was thoroughly removed, just to the point where it stops sputtering, and the solids in the bottom begin to brown.

If the unsalted butter has a fat content of 80% and clarified near 100%, then a 20% clarified to 80% unsalted ratio results in 84% fat.

I let the two warm to room temperature, then beat them thoroughly with an electric mixer to combine.

Immediately after beating I transferred the now very soft butter to parchment paper. Folding the paper over, I squished the butter into a nice slab for laminating, wrapped it tightly with the parchment, and put it in the fridge to chill until the dough is ready.

I'm still experimenting with the flavor of croissants made this way. Obviously this won't reproduce the complex flavors of a cultured butter.

However, the workability of this combination is great: no shattering even when chilled in the freezer, unlike 80% fat unsalted butter alone.