Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the longer sides to easily lift the bars out later.

In a large mixing bowl, whisk together the all-purpose flour, 1/2 cup granulated sugar, and 1/4 tsp salt.

Add the cold, cubed butter to the dry ingredients. Using a pastry blender or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs.

Press the mixture evenly and firmly into the bottom of the prepared 9x13 inch baking pan.

Bake the crust for 18-20 minutes, or until the edges are lightly golden brown.

While the crust is baking, prepare the topping. In a medium saucepan, combine the 1/2 cup unsalted butter, packed light brown sugar, and light corn syrup.

Heat over medium heat, stirring occasionally, until the butter is melted and the sugar is completely dissolved. Bring the mixture to a rolling boil, then reduce heat slightly and let it boil for 2 minutes, stirring constantly to prevent scorching.

Remove the saucepan from the heat. Carefully stir in the heavy cream, vanilla extract, and 1/4 tsp salt. Mix until the topping is smooth and well combined.

Stir in the toasted, chopped pecans.

Pour the warm butterscotch pecan topping evenly over the hot, baked shortbread crust in the pan.

Return the pan to the oven and bake for an additional 20-22 minutes, or until the topping is bubbly and set, and lightly browned around the edges.

Remove from the oven and place the pan on a wire rack to cool completely. This step is crucial for the bars to set properly and for the topping to firm up. Cooling may take 2-3 hours at room temperature, or you can refrigerate for about 1 hour once it's mostly cooled to speed up the process.

Once completely cooled, use the parchment paper overhangs to lift the entire slab of bars out of the pan. Place on a cutting board and cut into 16 or 24 squares, depending on your desired serving size.