Preheat your oven to 350°F (175°C) and prepare an 8x8 inch baking pan by greasing it and lining it with parchment paper.

In a mixing bowl, cream together 1 cup of softened unsalted butter and ¾ cup of superfine white sugar until the mixture is light and fluffy. Then, add 1 tablespoon of pure vanilla extract and mix until well combined.

In a separate bowl, whisk together 2 ½ cups of all-purpose flour and ¼ teaspoon of salt.

Gradually incorporate the dry ingredients into the butter mixture, mixing until just combined to prevent overworking the dough.

Press half of the dough into the prepared pan, setting aside the remaining half for the top layer. Cover the reserved dough and chill it in the fridge along with the pan for 15 minutes.

After chilling, bake the pan in the preheated oven for 10 minutes. The shortbread should appear slightly underbaked, which is fine as it will bake again with the filling.

Remove the pan from the oven and let it cool while you prepare the cinnamon filling.

In a small bowl, combine ¼ cup of melted unsalted butter, ½ cup of brown sugar, 1 teaspoon of cinnamon, and 1 teaspoon of vanilla extract, mixing until well blended.

Spoon the cinnamon filling onto the partially baked shortbread crust and spread it into an even layer.

Roll out the remaining shortbread dough or shape it into flat discs about ¼ inch thick. Place these discs on top of the filling to create a loose lid.

Return the assembled bars to the oven and bake at 350°F for 30 minutes, or until the edges start to brown. The center may look slightly underdone, which will result in a more tender shortbread. Check the bars at the 25-minute mark to avoid overbaking.

Once baked, remove the bars from the oven and let them cool completely to room temperature.

In a small bowl, mix together 1 cup of powdered sugar, 1 ½ teaspoons of pure vanilla extract, ½ teaspoon of cinnamon, and 3 ½ teaspoons of water until smooth.

Pour the icing over the cooled shortbread and spread it into an even layer. Allow the icing to set for about 15-20 minutes before slicing and enjoying.