Preheat the oven to 325 degrees F.
Completely line a 9 by 13-inch baking dish with aluminum foil (shiny-side down) and grease the insides.
Cream butter until soft. Add sugars, eggs, 1 at a time, and vanilla. Add flour and salt.
Melt chocolates in a double boiler and add baking soda.
Put 2/3 of the white batter into the baking dish. Add the melted chocolate mixture to the remaining batter. Mix thoroughly and spoon onto the white batter in the baking dish. Take a knife and cut through to marbleize. Bake until a toothpick inserted in the center comes out clean, about 40 minutes. Freeze the brownies when done.
Remove the brownies from the pan and peel off the foil. Cut in squares and serve. If you want, you can let them get soft and eat at room temperature.