1. Cut two 12-inch lengths of foil, folding the cut edges so that each sheet of foil is 7-1/2 inches wide. Press into an 8-inch-square baking pan so that ends of foil hang over the sides. Spray the foil with nonstick cooking spray.

2. Place the 2 ounces of unsweetened chocolate in a food processor or blender and process until finely chopped, almost powdery.

3. Add the semisweet chocolate chips to the chopped unsweetened chocolate in a large bowl. Add the baking soda and toss until baking soda is evenly distributed. Set aside.

4. Using a Dutch oven, combine the sweetened condensed milk and salt and cook over medium heat, stirring frequently with a wooden spoon, until bubbles appear along edges of the pan. Continue cooking, stirring constantly, until the mixture thickens beginning to look caramel colored, about 4 minutes.

5. Remove pan from heat and add the chocolate mixture and vanilla extract. Using your wooden spoon, stir until the chocolate is completely melted and the mixture is smooth, about 5 minutes.

6. Stir in the walnuts and continue to stir until the fudge is no longer shiny and resembles stiff cookie dough. Transfer fudge to the foil-lined pan and spread evenly using a spatula. Place a sheet of wax paper on top and press firmly to distribute the fudge evenly. Chill in refrigerator for at least one hour. Using the foil lining, carefully lift the fudge from the pan and cut into squares.