Make shortbread crust according to directions for a 9x13-inch baking pan. Set aside to cool as you prepare the caramel.
Salted Caramel Layer
To prepare the caramel, mix all ingredients through half and half in a heavy bottomed, 3-4 quart saucepan.
Cook over low to medium heat until the caramel has reached a warm golden brown color and has reached the 250º F on a candy thermometer, or the hard ball stage.
Remove from the heat, add vanilla extract, and beat until creamy with a wooden spoon.
Pour onto prepared shortbread crust and allow to cool for about 10 minutes. Cover with plastic and place into the refrigerator for 1-3 hours to overnight to allow caramel to firm.
Store in the refrigerator to keep the caramel firm, recommended.
Remove from the refrigerator about 20 minutes before ready to serve.
Notes
Prep time does not include time for chilling salted caramel shortbread bars in the refrigerator for caramel to firm before serving.