Measure 4 cups of flour and 2 tsp salt in a large mixing bowl.
Cut in 100 g cold butter with a pastry blender until it resembles coarse crumbs.
Pour 1 1/4 cups water into a well in the center of the flour.
Mix and knead for a few minutes until the dough becomes smooth.
Chill dough for 30 min. Wrap in cling wrap.
Mix 400 g room temperature butter and 1/2 cup flour together. Shape into a 9" square by beating it with a rolling pin in between 2 cling wrap pieces on a cutting board. Chill in the fridge while you roll out the dough.
Roll the dough out to a 12" square. Place butter in a diamond shape on the dough.
Close the dough around the butter like an envelope and seal the edges so the butter is fully enclosed.
Use flour as needed to roll the dough into a rectangle shape until its ~ 1/4" thick. Fold the dough like a letter. Fold one side over the center. Then fold other other side over the center creating 3 layers of even size.
Roll out the dough into a rectangle again and repeat the letter folding process. Chill for 30 min.
Repeat this process until you have completed 6 folds, chilling for 30 min in between every 2 folds.
Your puff pastry is ready to be used. You can roll it out until it's about 1/4 " in thickness to use. You can cut the dough in half and freeze it if you're not going to use the whole thing. 1/2 dough will roll out to 10" x 24". Pieces of puff pastry that are 10"x 9" should be baked for 18-25 min at 400 degrees F. Pieces that are 5"x5" can be baked for 15-20 min at 400 degrees F.