Set the oven rack to the middle position. Pre-heat to 325ºF (165ºC).
Lightly butter and line an 8x8 baking pan with parchment leaving a 1-inch overhang on each side. Set aside.
Using a hand mixer, mix the butter, sugar, vanilla, and salt, on low speed to combine, then increase to medium speed. Mix until light and creamy, about 2 minutes. Scrape down the sides of the bowl.
Add the flour and mix on low until a hydrated and crumbly dough is formed. Do not overmix.
Add the dough to the baking pan. Lightly press it in until evenly distributed - getting the dough into the corners. Place a piece of plastic wrap on top, and smooth out - use a spatula to flatten the surface.
The dough will have creeped up the sides, so just fold the parchment over as shown to prevent over browning.
Lightly prick the surface, about 30 times, evenly spaced. Cover and refrigerate for 30 mins.
Bake until the surface and edges are golden, about 30 to 35 mins.
Cool for 5 mins. While the biscuits are still hot, lightly score your cookies with a sharp knife, as a guide before cutting into them to ensure rectangles are even, then cut.
While cookies are warm, sprinkle the surface with 1 tbsp sugar so it sticks.
Let it cool completely in pan.
Lift the biscuits out using the parchment paper. Re-slice the lines if needed.