Rich, moist, delicious. This was a recipe my mom always made. She used a lot more flour and less sugar. My kids love when I bake, and I took this recipe and played around with it, changing it to be chewy and not cake-like. They also liked the vanilla and chocolate mix. She would use unsweetened chocolate but my boys prefer this combo. After weeks of changing the recipe, this is what we came up with to being their favorite.
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.