Heat the oven to 350 degrees. Line a 13-by-9-inch baking pan with parchment paper, leaving a 2-inch overhang on two sides. Butter any of the exposed pan.
In a large bowl, beat together the butter, flour, brown sugar and salt with an electric mixer on medium speed until the crumbs are evenly moistened. (The mixture will be sandy.)
Tip into the prepared pan and press down into an even, flat layer using a flat-bottomed measuring cup or an offset spatula. Sprinkle the pecans evenly over the crust.
In a medium saucepan, melt the butter, sugar and salt over medium-high heat. Bring to a simmer and cook for 1 minute. Remove from the heat and stir in the vanilla. Pour the caramel evenly over the crust and pecans.
Bake until the nuts smell toasty and the caramel is bubbling evenly over the entire surface, about 20 minutes. Transfer to a rack and then carefully sprinkle the chocolate chips evenly over the top. Let stand for 5 minutes and then carefully spread the chocolate out into an even layer.
Let stand at room temperature for at least 4 hours or until completely cooled.
Transfer to a cutting board and cut into 1-by-2-inch bars. Store at room temperature in an airtight container for up to 1 week or freeze for up to 3 months. (If freezing the bars, let them thaw before serving.)