Preheat oven to 350°F (175°C). Line the bottom and sides of a 9-in (23 cm) square baking pan with parchment paper. In a large mixing bowl, mix together the flour, cocoa powder, brown sugar, and salt. Drizzle in the melted butter, then use a fork to mix the butter into the dry ingredients. (The crust mixture should hold together when pressed between two fingers.) Transfer the crust mixture to the prepared baking pan, then press down to form an even, thin crust. Bake for 15 minutes, then remove from the oven and set aside. Keep the oven on.
For the filling
In a large mixing bowl, mix together the brown sugar, flour, and salt. Add the eggs and mix and whisk vigorously to incorporate. Whisk in the vinegar and vanilla extract, then stir in the butter and pecans.
Pour the filling over the pre-baked crust. (The crust can and should still be warm.) Sprinkle the chocolate chips over the top. Bake the bars for about 40 minutes, or until the filling is set most of the way to the center, with a small section in the center that is still "jiggly." (The filling will finish setting as it cools.)
Transfer the pan to a wire rack and let the bars cool completely, at least 1 hour. Slice into 16 bars.
STORAGE: Store the chocolate pecan pie bars in an airtight container at room temperature for up to 3 days. You can also refrigerate them for 1 week, but always make sure to return to room temperature before eating.
Recipe Credit: Filling adapted from a pecan pie recipe by King Arthur Flour.