For the crust: Heat the oven to 350 degrees F and arrange a rack in middle. Line an 8 by 8-inch baking dish with aluminum foil, leaving a few inches of overhang on each side.
Combine the pecans, butter, flour, sugar, and salt in bowl of a food processor and pulse until the mixture is well combined and it resembles coarse meal. Press the mixture evenly into the prepared baking dish and bake until lightly golden, about 25 to 30 minutes. Remove from the oven and immediately make the filling.
For the filling: Whisk together the sugar, molasses, corn syrup, butter, rum if using, vanilla, salt, and eggs in a small bowl until well combined. Add the pecans and stir to coat. Pour the pecan mixture over the hot crust and return the baking dish to the oven.
Bake until top is browned and set, about 25 to 30 minutes. Remove from the oven and let cool completely before removing from dish and cutting into 1-inch squares.