For a long time in proofing croissants, I've used levain rather then adding commercial yeast. Call it purity or foolishness, I call it a challenge. If you don't remember, here's proof of the batch that blew open the doors to my fermentation nirvana. And if you don't want to click, let's just say I ended up with the most perfect croissants in human history. Okay, they were the best I had ever personally made. 

Subsequently, my success failed to replicate itself, with me repeatededly proving incapable of remaking magic. It was as if I had forgotten some critical step. Was it underproofing, excessive acidity, weak mixing? It was a mystery, leaving me in a tail spin of croissant insecurity, despondently facing the fact that there would be no matrix of layers for this baker.

To cut to the chase, I dug in and researched and rolled and even thought of throwing in my baker's towel. Was it back to commercial yeast for me? Almost, but first I decided to seek counsel from Aidan Chapman and Dirk Braeckman, both of them bringing up the idea "everything cold," meaning from fermentation to butter. Between the big chill and the big bang hot oven, voila… sproink! My layers returned, just four the fourth of July holiday weekend.