Total time: 25 minutes
10 tablespoons unsalted butter, plus more for greasing
4 ounces bittersweet chocolate
4 ounces semi-sweet chocolate
2 large eggs
1 cup granulated sugar
2 teaspoons vanilla extract
¼ teaspoon kosher salt
1 cup cake flour, sifted
1. Heat the oven to 400°F|200°C.
2. Grease a 9-by-13-inch baking pan with butter and line with parchment paper. Grease parchment paper and set pan aside.
3. Pour enough water into a 4-quart saucepan that it reaches a depth of 1-inch. Bring to a boil and reduce the heat to maintain a simmer. Combine the butter and chocolate in a medium bowl and set the bowl over the saucepan. Cook, stirring, until melted and smooth, about 5 minutes. Remove from heat and set aside.
4. Whisk the eggs together in a large bowl. Add the sugar, vanilla, and salt and whisk to combine. Stir in the chocolate mixture and fold in the flour.
5. Spread the batter evenly into the prepared pan and bake until the brownie forms a thin skin on top but the batter is still loose in the center, about 18 to 20 minutes. Transfer the pan to a rack and let cool completely before slicing.