Maybe it's the Irish in me, but I have a major thing for shortbread and have made it every which way imaginable. My father (where I get my Irish from) is obsessed with Pecan Pie so I combined the two desserts and it's now in regular rotation at Thanksgiving.
(or Maple syrup)
(or light molasses)
optional (or coffee liqueur or bourbon)
For the crust: Heat the oven to 350°F and arrange a rack in middle. Butter an 8 by 8-inch baking dish then line with parchement paper or aluminum foil, leaving a few inches of overhang on each side. Place the lined baking dish in the freezer until ready to use.
Combine 3/4 cup of the pecans with the 1/2 cup of the granulated sugar, and 1 teaspoon of the salt in a food processor. Pulse until the nuts are a coarse meal. Add the flour and process until it's a fine meal and evenly mixed. Cut 8 tablespoons of the butter into small cubes, add it, and pulse until the mixture comes together in a clump.
Using the back of a lightly floured measuring cup, press the mixture evenly into the prepared baking dish and place in the freezer for 10 minutes to cool briefly. Bake until lightly golden, about 25 to 30 minutes. Remove from the oven and let cool at least 10 to 15 minutes before continuing. Meanwhile, make the filling.
The shortbread can be made up to 2 days ahead of time. Cover and store at room temperature until ready to use.
For the filling: Melt the remaining 4 tablespoons of butter and roughly chop the remaining pecans. Whisk together the remaining 3/4 cup brown sugar, corn syrup, maple syrup, melted butter, dark rum (if using), vanilla, remaining 1/2 teaspoon of salt, and eggs in a small bowl until well combined. Add the pecans and stir to combine evenly. Pour the pecan mixture over the crust and return the baking dish to the oven.
Bake until top is well browned, slightly puffed, and set (it will no longer jiggle when the pan is shaken), about 35 to 45 minutes. Remove from the oven and let cool completely before removing from dish. Using a sharp knife, cut into 1-inch squares and serve at room temperature or warmed slightly.
Pecan pie bars can be made up to 4 days ahead of time and stored at room temperature in an airtight container.