The cookies are softer and more tender when made with unbleached flour that has a protein content of about 10.5 percent. Pillsbury or Gold Medal works best; King Arthur flour has a higher protein content (around 11.7 percent) and will result in slightly drier, cakier cookies. Do not discard the butter wrappers; they have just enough residual butter on them for buttering the bottom of the drinking glass used to flatten the dough balls. To make sure the cookies are flat, choose a glass with a smooth, flat bottom. Rolled into balls, the dough will keep in the freezer for up to 1 week. The baked cookies will keep in an airtight container for up to 5 days.