1. Preheat oven to 325°. Coat a 9×13 baking dish with nonstick spray. Set aside.
2. In the bowl of your stand mixer fitted with the paddle attachment, mix the butter, granulated sugar, and powdered sugar together on medium speed for 2 minutes. Add the vanilla and mix until combined. Turn the mixer to low and add in the flour, mixing until a soft dough forms.
3. Press one-third of the dough evenly into the bottom of the prepared pan. Wrap the remaining dough in plastic wrap and chill in refrigerator.
4. Bake for 20 minutes, or until the edges are a pale golden brown. Transfer the pan to a wire rack and let cool about 15 minutes.

To Make the Filling:

1. While the bottom crust is baking and the remaining dough is chilling, make the caramel filling. Place the unwrapped caramels in a microwave-safe bowl. Add the milk or cream. Microwave on high for 1 minute. Remove from the microwave and stir until smooth. If caramels are not completely melted, microwave on high for 30-second intervals, stirring after each interval, until smooth.
2. Once the caramel is melted add in your ½ teaspoon of vanilla and stir until combined.
3. Pour the caramel filling over the cooled crust. If you are going to salt the caramel sprinkle it on caramel layer now.
4. Remove the remaining chilled dough from the refrigerator and crumble it evenly over the caramel.
5. Return the pan to the oven and bake until the filling is bubbly and the crumbled shortbread topping is firm and lightly golden, about 25 – 30 minutes.
6. Let cool completely before cutting into squares.