• Make-Ahead
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Ingredients

1. 1 1/4 pounds (5 sticks) unsalted butter, softened
2. 1 pound unsweetened chocolate, chopped
3. 5 1/2 cups sugar
4. 16 large eggs
5. 2 tablespoons pure vanilla extract
6. 1 1/2 teaspoons kosher salt
7. 3 3/4 cups all-purpose flour
8. 1/2 pound bittersweet chocolate, chopped into 1/2-inch pieces
9. 1/2 pound white chocolate, chopped into 1/2-inch pieces
10. 1/2 pound milk chocolate, chopped into 1/2-inch pieces

Directions

1. 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.
2. 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.
3. Transfer the brownies to a wire rack to cool completely before cutting into squares.