The difference between using very cold butter and room temperature butter when making puff pastries and croissants lies in the technique used to create layers of butter within the dough, which is essential for achieving the desired flaky and light texture in these pastries.
Very Cold Butter: When making puff pastry or croissants with very cold butter, the butter is solid and firm. The dough is rolled out first, and then a block of cold butter is placed in the center. The dough is then folded over the butter, encasing it completely. By keeping the butter cold, it remains in a solid state during this process, preventing it from mixing with the dough. The dough is then rolled and folded multiple times in a technique known as "laminating." This creates distinct layers of butter and dough, resulting in a flaky texture when baked.
Room Temperature Butter: If room temperature butter is used instead, it would be much softer and could easily mix with the dough when the pastry is being formed. This would result in a more homogenous mixture, and the layers of butter and dough wouldn't form as distinctly. The final pastry might be more like a regular dough, lacking the characteristic flaky and delicate texture of puff pastries and croissants.
In summary, the use of very cold butter is crucial for creating the distinct layers and achieving the signature flakiness of puff pastries and croissants. It allows the butter to remain separate from the dough during the laminating process, resulting in light, crispy, and airy pastries when baked.