Makes 12-18 bars
Adapted from Thomas Keller
3/4 cup all-purpose flour
Preheat the oven to 350 degrees Fahrenheit. Butter and flour a 9-inch square baking pan in metal or glass or use a silicone one. Note, if you have an 8-inch square pan you will have to increase the baking time by about 10 minutes. Set aside the pan.
Sift together the flour, cocoa powder, and salt; set aside.
Microwave half the butter in a medium bowl until melted. Add the remaining butter to the melted butter and stir to melt further. The butter should look creamy, with small bits of unmelted butter, and be at room temperature.
In a bowl of a stand mixer fitted with the paddle, mix together the eggs and sugar on medium speed for about 3 minutes, or until thick and very pale and resembling custard in thickness. Mix in the vanilla. On low speed, add about one-third of the dry ingredients, then add one-third of the melted, cooled butter, and continue alternating the remaining flour and butter. Add the chopped chocolate and mix to combine. (The batter can be refrigerated for up to 1 week.)
Spread the batter evenly in the pan. Bake for 40 to 45 minutes, until a cake tester or wooden skewer poked into the center comes out with just a few moist crumbs sticking to it. If the pick comes out wet, test a second time, because you may have hit a piece of chocolate chip; then bake for a few more minutes longer if necessary. Cool in the pan until the brownie is just a bit warmer than room temperature.
Run a knife around the edges if not using a silicone mold, and invert the brownie onto a cutting board. Cut into 12 thick rectangles. Dust the tops with powdered sugar just before serving.
The brownies can be stored in an airtight container for up to 2 days.