1/2 cup (1 stick) unsalted butter, cut into pieces
6 ounces bittersweet chocolate, chopped
1 1/2 cups sugar
3 large eggs
1/4 cup unsweetened cocoa powder
1/2 teaspoon salt
1/2 cup plus 2 tablespoons all-purpose flour, sifted
Preheat oven to 350 degrees. Line an 8-by-8-inch baking pan with parchment paper, leaving an overhang on all sides; set aside.
In a medium bowl, combine butter and chocolate and place over (but not touching) simmering water; stir occasionally until melted and smooth. Remove bowl from heat and whisk in sugar. Add eggs, one at a time, until incorporated. Add cocoa powder and salt. Fold in flour and mix until combined.
Pour batter into prepared baking pan and transfer to oven; bake until set and a cake tester inserted into the center comes out with crumbs but is not wet, 35 to 40 minutes. Remove from oven and let cool for 15 minutes before removing brownies from pan. Let cool completely on a wire rack before cutting. Brownies can be stored in an airtight container for up to 3 days.