Mix all the ingredients (either with your hands or with the paddle attachment for about 1min)
Cover with a plastic wrap and let it rest at room temperature for 15h.
You can knock back the poolish from time to time if you want, and after 15h you should get something like that
Following the puff pastry blog post's instructions, mix the croissant poolish with the croissant dough packet (détrempe) ingredients and make a puff pastry dough using two double folds.
It is important to use spring or mineral water (without too much sodium) and not tap water. Tap water can be unreliable for any leavened dough (e.g. for bread) because its property can be unpredictable (from one to another, or even throughout the year): chlorine and chloramine can lessen your yeast growth, the water hardness can also be a significant issue etc.
For those reasons, it's far easier and predictable to work with the same spring water all the time. Leavened doughs are tricky enough not to add unnecessary difficulties.
Détrempe and croissant dough notes
Be sure to follow the puff pastry blog post. You can also substitute part of the milk for spring water, e.g. 50%-50%, or even 100% water. The more water you substitute for, the crispier and less brioche-like it will be. It really depends on your taste and the texture you want to get.
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Let me know how you liked it and stay tuned for the next blog post/recipe which will explain how to properly shape and bake them for best results!