Cream the butter and brown sugar together in a large bowl with a mixer. Add 3 to 3 3/4 cups flour, one cup at a time, mixing well after each cup. Sprinkle your work surface with the remaining flour. Knead the dough a few times until it comes together, then roll it out with a rolling pin so that it's about 1/4 to 1/2 an inch in thickness.
Use either a 2-inch cookie cutter or a shortbread cooking cutter to cut your cookies into desired shape. Prick with fork or a toothpick so make little holes in the cookies and place on ungreased baking sheets.
Bake at 325 degrees F (165 degrees C) for 20 to 25 minutes. Do not let them get brown. Remove from the oven. Do not let them get brown. Remove from the oven. Let the cookies cool on baking sheets for 5 minutes before transferring them to a rack to finish cooling.
Instead of cutting the cookies into desired shapes you can pat the dough into an ungreased 17.88 x 12.88 x 1.06 inches baking sheet lined with parchment paper hanging off the sides to make it easy to lift out the cookies. After you bake them, remove them from the oven and lift out the cookies from the sheet using the parchment paper, then cut into desired shapes.
If using salted butter, there's no need to add additional salt to your cookies.
These cookies freeze incredibly well. To freeze, place shortbread cookies in an airtight container and place in the freezer for up to 3 months.