Preheat your oven to 350°F (175°C). Line the baking pan with parchment paper, leaving some overhang for easy removal later.
In a mixing bowl, combine the flour and granulated sugar. Add the softened butter and mix until the mixture resembles coarse crumbs.
Press the mixture firmly into the bottom of the prepared baking pan to form an even layer. Bake for 15 minutes in the preheated oven, or until lightly golden.
While the crust is baking, prepare the filling. In another mixing bowl, whisk together the brown sugar, corn syrup, melted butter, eggs, vanilla extract, and salt until well combined.
If using, stir in the chopped nuts into the filling mixture.
Once the crust is out of the oven and still hot, pour the filling over it, spreading evenly.
Return to the oven and bake for an additional 15-20 minutes, or until the filling is set and slightly bubbling.
Remove from the oven and allow to cool completely in the pan on a wire rack.
Once cooled, lift the parchment paper to remove the squares from the pan.
Cut into 12 squares and serve.