Set out an electric stand mixer. Add the softened butter and brown sugar. Cream on high until light and fluffy, about 3-5 minutes. (Don’t skimp on the time!)
Scrape the bowl with a rubber spatula, then turn the mixer on low. Add the vanilla extract, then slowly add in the flour. Once combined, turn off the mixer as to no over-mix the dough.
Press the dough into a rectangle and wrap well in plastic. Chill for at least 1 hour.
When ready to bake, preheat the oven to 325 degrees F. Line 2 large baking sheets with parchment paper.
Thoroughly flour a clean work surface. Place the dough in the center of the floured area and roll it out into an even sheet, ½ inch thick. Use as much flour as needed to keep the dough from sticking to the counter or rolling pin.
Use a large chef’s knife (or pizza cutter) to cut the shortbread dough into 2 ½ X 1 inch pieces. Carefully move the pieces to the baking sheets, spreading them 1 ½ inches apart.
Use a fork to poke holes over the surface of each cookie. This helps release steam so the cookies hold their shape better.
Bake for 20-22 minutes, until golden around the edges. Sprinkle the top of each cookie with Fleur de Sel while they are still warm. Cool for 5 minutes on the baking sheets before moving.
Keep shortbread cookies stored in an airtight container at room temperature for up to a week.Scottish Shortbread can be frozen for up to 3 months.If you stack them, place a piece of parchment paper between the layers to keep the cookies freshest.