Gooey Caramel Butter Bars

To make the crust:

• In a large bowl, combine the butter and sugars. Using an electric mixer set at medium speed, beat together until creamy. Add the vanilla and salt 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. Pat the remaining dough into a flat disk and wrap in plastic wrap and refrigerate until firm, at least 30 minutes.
• Position a rack in the middle of the oven and preheat to 325F. Bake until firm and the edges are a pale golden brown, 20 to 25 minutes. Transfer to a wire rack and let cool.
• 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, rum (if using), and salt. 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.
• Sprinkle the nuts, if using, over the bottom crust. Pour the caramel filling over the nuts, using a small metal spatula to nudge the filling evenly over the crust. 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 30 minutes. Transfer to a wire rack and let cool completely.
• Use a sharp knife to cut the bars evenly into 15 large squares. Remove the bars from the pan with a metal spatula and, if desired, cut in half on the diagonal to form 30 triangular bars. Dust with confectioners’ sugar, if desired.
• The bars will keep, covered tightly at room temperature, for about 1 week, or in the freezer for up to 1 month.