Warm the milk to 110 degrees F and pour it over the yeast and 1/2 teaspoon of the sugar. After it foams, add the egg, flour, the remaining sugar, and the salt. Knead a few minutes. Chill for 1 hour. Arrange butter slices next to each other between sheets of parchment and rolll very thin, approximately 6-by-16 inches.
Roll out the chilled dough on a lightly floured surface into a 16-by-24-inch rectangle. Arrange teh butter on one half of the dough, lenthwise, leaving space along the border. fold the other half of the dough over this and press to seal well so no butter seeps out during baking. Fold into thirds (like a business letter), wrap in plastric, and refrigerate for at least 30 minutes.
Rotate 90 degrees from teh position the dough had been in before chilling and repeat the process, rolling it out into a long rectangle and folding it into thirds, then refrigerating for an additional 30 minutes
Do this once more. Refrigerate until you're ready to use the dough, once again making sure to give it at least 30 minutes to get the butter cold enough.