Line a 8 x 8 baking dish with aluminum foil; lightly spray foil with cooking spray.
In a medium size, bowl mix together the chocolates, salt, and baking soda until well combined, and then stir in the condensed milk.
Place 2 cups of water in a 4-quart saucepan and heat until simmering. Place the bowl in the pan to create a double boiler. Stir chocolate constantly until most of the chocolate is melted, but a few small pieces remain, remove from heat and continue stirring until all the chocolate is melted; and then fold in the nuts.
Using a rubber spatula layer the chocolate mixture over the bottom of the baking dish; refrigerate at least 2 hours before cutting, and serving.
Wrap leftover fudge with plastic wrap to keep it from drying out and store it in the refrigerator, or freezer (can freeze for up to 3 months).