With the mixer running on low speed, add the butter one tablespoon at a time, allowing each piece to fully absorb before adding the next. This process can take 10 to 15 minutes, and the dough may look like it is falling apart at first, but it will come back together. By the end, the dough should be glossy, elastic, and very soft. Check gluten development: Pinch off a small piece of dough and gently stretch it between your fingers. If it stretches into a thin, translucent sheet without tearing easily, the gluten is well developed. If it tears quickly, continue kneading for a few more minutes and test again.