In the bowl of a stand mixer fitted with the paddle attachment or with a hand-held mixer, beat the butter and both sugars until creamy, about 3 minutes. Add the vanilla and salt and beat until combined. Add the flour and beat on low speed until a smooth, soft dough forms.

Press one-third of the dough evenly into the pan to form a bottom crust. Pat the remaining dough into a flat disk, wrap in plastic wrap, and refrigerate until firm, at least 30 minutes.

Bake until firm and the edges are a pale golden brown, 20-25 minutes. Transfer to a wire rack to cool.

Place the unwrapped caramels in a microwave-safe bowl. Add the half and half, vanilla, liqueur (if using), and salt. Microwave on high for 1 minute. Remove from the microwave and stir until smooth. If the caramels are not completely melted, microwave on high in 30-second intervals, stirring after each interval, until smooth.

Sprinkle the nuts (if using) over the bottom crust. Pour the caramel filling over the nuts using an off-set spatula to spread the filling evenly. Remove the remaining 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 30-35 minutes. Transfer to a wire rack to cool completely.