Preheat your oven to 325°F (165°C).
In a large mixing bowl, cream together the softened unsalted butter and packed brown sugar until light and fluffy, about 3-5 minutes.
Sift in the all-purpose flour and salt, and gently fold into the butter mixture without overmixing.
Knead the dough lightly until smooth, adding a teaspoon of water if too crumbly.
Roll out the dough to about 1/2 inch thickness and cut into rectangles or shapes.
Line a baking sheet with parchment paper and arrange the shortbread pieces with space in between.
Bake for 18-20 minutes until edges are lightly golden. Monitor closely to avoid overbaking.
Cool on the baking sheet for 5 minutes, then transfer to a cooling rack to cool completely.