Preheat the oven to 325°F.
In a large mixing bowl, cream the butter and brown sugar until light and fluffy, about 5–7 minutes.
Add 3 3/4 cups of flour and mix until combined.
Transfer the dough onto a lightly floured surface.
Knead the dough for about 5 minutes, adding enough remaining flour to create a soft dough.
Place the dough on a sheet of parchment paper and roll it into a 16x9-inch rectangle.
Transfer the parchment onto a baking sheet.
Cut the dough into 3x1-inch strips.
Prick each Scottish Shortbread cookie several times with a fork.
Refrigerate the dough for at least 30 minutes or overnight.
Separate the cookies and place them 1 inch apart on ungreased baking sheets.
Bake for 20–25 minutes or until lightly browned.
Transfer the cookies to wire racks and cool completely before serving.