Preheat the oven to 350°F. Line a light-colored 9×9-inch metal baking pan with parchment paper, leaving enough overhang to lift the bars from the pan.
Beat the softened butter and powdered sugar together until smooth and creamy.
Mix in the vanilla extract.
Add the flour and salt and mix just until a soft, crumbly dough forms.
Press the shortbread dough evenly into the bottom of the prepared pan. Prick the surface several times with a fork.
Bake for 20–24 minutes, until the edges are lightly golden. Place the pan on a wire rack while you prepare the caramel.
Spread the chopped pecans and pecan halves on a baking sheet. Toast them in the oven for 5–7 minutes, stirring once, until fragrant. Set aside.
Add the butter, dark brown sugar, sweetened condensed milk, corn syrup, vanilla extract, and fine sea salt to a heavy-bottomed saucepan.
Cook over medium-low heat, stirring constantly, until the butter has melted and the mixture is smooth.
Increase the heat to medium and bring the mixture to a gentle boil. Continue cooking and stirring constantly, scraping the bottom and sides of the saucepan.
When the caramel reaches approximately 218°F, remove the saucepan from the heat.
Transfer approximately ¼ cup of the caramel to a small heatproof bowl and set it aside for the drizzle.
Return the saucepan to medium-low heat. Continue cooking and stirring until the remaining caramel reaches 228°F.
Immediately pour the thick caramel over the baked shortbread and spread it into an even layer.
Sprinkle the toasted chopped pecans evenly over the caramel and press them down gently. Arrange the 16 pecan halves in a 4-by-4 pattern so each finished bar will have one.
Let the bars cool at room temperature for approximately 30 minutes. Refrigerate for at least 2½ hours, until the caramel is completely firm.
Use the parchment paper to lift the chilled bars from the pan.
Warm the reserved caramel in the microwave in 5-second intervals until it is pourable. Drizzle it over the pecan topping.
Return the bars to the refrigerator for 10–15 minutes, until the drizzle is set enough to cut.
Cut the slab into 16 rectangular bars, following the placement of the pecan halves. For cleaner cuts, use a large sharp knife and wipe it between each cut.