Preheat oven to 325 degrees.  In a large bowl, combinebutter and sugars. Beattogether until creamy with electric mixer. Add vanilla and beat until combined. Sift flour into the mixture and beat until a soft dough forms. Spray a 9×13 inch baking with non stick cooking spray. press 1/3 of the dough evenly into the pan to form a bottoms crust. Wrap remaining dough in plastic wrap and chill in refrigerator. Bake crust until firsm and the edges are a pale golden brown (approx. 20 minutes.) Cool on wire rack to let cool for 15 min. 

While crust is baking, make caramel filling. Place the unwrapped caramels inn a microwave safe bowl. Add milk. Microwave for 1 min. Remove from the microwave and stir until smooth. If caramels are not completely melted, microwave on high for 30 sec intervals, stirring after each interval until smooth. Once the caramel is melted add in your 1/2 tsp vanilla and stir. Pour caramel over crust. If you are going to use coarse sea salt, sprinkle it over the caramel layer now. 

Remove the maining chilled dough and crumble evenly over the caramel. Return the pan to the oven and bake until hte filling is bubbly and the crumbled shortbread topping is firm and lightly golden, about 25-30 min. Let cool before cutting into squares.