In a large bowl, combine the butter and sugars. Using mixer on medium speed, beat together until creamy. Add the vanilla and beat until combined. Sift the flour into the butter mixture and beat on low speed until a smooth soft dough forms. Spray a 9×13 inch baking pan lightly with non-stick cooking spray. Press one-third of the dough evenly into the pan to form a bottom crust. Preheat to 325°F Bake until firm and the edges are a pale golden brown approx 20 minutes. Transfer pan to a wire rack and let cool about 15 minutes. 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 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 (I did the melting in a saucepan stovetop since I don't microwave). Pour the caramel filling over the crust. If you are going to salt the caramel sprinkle it on caramel layer now. Remove the remaining chilled dough from the refrigerator and crumble it evenly over the caramel. 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. Let cool before cutting into squares.