Preheat oven to 350°F. Grease a 9- or 10- inch round springorm pan.
Using an electric mixer on medium speed, beat the butter and brown sugar until light and fluffy.
Reduce mixer speed to low. Add the flour and salt, and mix just until combined and the mixture forms crumbs.
Transfer the crust mixture to the prepared pan. Press evenly and firmly into the bottom of the pan.
Bake for 15 to 20 minutes, or until golden brown. Set aside.
Place the condensed milk and chocolate in a heavy saucepan. Cook over medium heat, stirring frequently, for 8 to 10 minutes, or until the mixture is smooth and thickened.
Spread the filling evenly over the crust. Sprinkle the nuts over the top of the filing, and press down lightly. Refrigerate for at least 2 hours, or until the filling is set. Serve chilled or softened.