Preheat the oven to 350F. We use a 9-inch square silicone mold, because it keeps the edges from overcooking; if you use a metal or glass baking pan, butter and flour it. Set aside.

Sift together the flour, cocoa powder, and salt; set aside

Melt half the butter in a small saucepan over medium heat, stirring occasionally. Put the remaining butter in a medium bowl. Pour the melted butter and stir to melt the butter. 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. Mix in the vanilla. On low speed, add about one-third of the dry ingredients, then add one-third of the butter, and continue alternating the remaining flour and butter. Add the 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 rectangles. Dust the tops with powdered sugar just before serving.