Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper for easy removal.

In a medium bowl, combine the flour, granulated sugar, and salt. Mix well.

Add the softened butter to the dry ingredients and blend until the mixture resembles coarse crumbs.

Press the crumb mixture evenly into the bottom of the prepared baking dish to form the crust. Bake for 15 minutes or until lightly golden.

While the crust is baking, prepare the filling. In a large bowl, whisk together the brown sugar, corn syrup, eggs, and vanilla extract until smooth.

Stir in the chopped nuts and raisins if using.

Once the crust is done, remove it from the oven and pour the filling mixture over the hot crust, spreading it evenly.

Return the dish to the oven and bake for an additional 25-30 minutes, or until the filling is set and slightly puffed.

Allow the squares to cool in the pan for at least 30 minutes before cutting into squares. For best results, chill in the refrigerator for an hour before serving.