A chewy brown sugar–pecan toffee layer spread over shortbread and topped with chocolate. These bars are best made a day ahead.
Preheat the oven to 350°F. Grease an 8-inch square baking pan and line with parchment paper so it extends over 2 sides.
Prepare the dough for:
Press the dough firmly into the bottom of the pan to form a smooth, even layer. Bake the dough for 15 minutes. Set aside on a rack (and leave the oven on).
Combine in a medium heavy saucepan and bring to a boil over medium heat, stirring frequently:
Boil, uncovered, for 3 minutes. Remove from the heat and stir in:
Spread the nut mixture evenly over the baked crust. Return the pan to the oven and bake until bubbling, golden brown, and just slightly darker at the edges, about 20 minutes. Place the pan on a rack and sprinkle over the top:
Let stand until the chocolate chips partially melt, then spread evenly with a table knife or offset spatula. Sprinkle over the top:
Let the chocolate cool until beginning to set but still slightly soft. Cut into 24 bars, then let cool completely in the pan before removing the bars.