How to Make It
Preheat the oven to 300°. Line the bottoms of two 9-by-13-inch baking pans with parchment paper. In a large saucepan, melt the butter with the unsweetened chocolate over low heat, stirring frequently, until melted. In a large bowl, whisk the sugar with the eggs, vanilla and salt. Add the melted chocolate and whisk until smooth. Add the flour and whisk until incorporated. Stir in 5 ounces of each of the chopped chocolates. Spread the batter in the prepared pans.
Place the remaining chopped chocolates separately in 3 small microwave-safe bowls and melt. Using a spoon, drizzle the melted chocolates over the batter. Using a table knife, make swirls in the batter for a marbled effect. Bake the brownies for about 35 minutes, rotating the pans halfway through, until the tops are shiny and the brownies are set.
Transfer the brownies to a wire rack to cool completely before cutting into squares.