Cream together your butter and brown sugar, making sure that there are no lumps in your brown sugar.
Add in the icing sugar and combine.
Add in the salt and flour slowly, with the mixer on low - this is not whipped shortbread after all- until the dough is completely mixed together.
Pat the cookie dough into an 8 x 8 wax paper/parchment paper lined pan.
Place in your freezer for 15-20 minutes.
Remove from the freezer and cut into squares. Place on a parchment lined baking sheet and decorate as you wish. I love half a candied cherry on top, it's the perfect size.
Bake at 325 degrees for 20-25 minutes, until the cookies are lightly browned and dried out.
Remove and cool on sheets for several minutes, then remove and cool completely on baking racks.