Preheat oven to 350°F (175°C). Position a rack in the center of the oven.

Butter a 9-inch baking pan. Line the pan with a sheet of parchment paper so there is about a 2-inch overhang on two opposite sides. Lightly butter the top of the parchment.

In a medium bowl, sift together the all-purpose flour, unsweetened alkalized cocoa powder, and kosher salt. Set this dry mixture aside.

Put half of the butter pieces in a small saucepan or a microwave-safe bowl and melt until fully liquid. Place the remaining butter pieces in a medium heatproof bowl.

Pour the hot melted butter over the butter pieces in the medium bowl. Stir continuously until almost all the butter pieces have melted and the mixture looks mostly smooth and creamy. Stir in the vanilla extract.

In the bowl of a stand mixer fitted with the paddle attachment (or using a large mixing bowl and electric hand mixer), combine the eggs and granulated sugar. Beat on medium speed until the mixture is thickened and pale in color, about 3–5 minutes.

Reduce the mixer speed to low. Add the sifted dry mixture and the butter mixture in three additions each, alternating between the dry and the butter: add one-third of the dry mixture, mix briefly; add one-third of the butter mixture, mix briefly; repeat two more times until fully combined. Scrape the bowl as needed so everything is evenly incorporated.

Remove the bowl from the mixer and fold in the semisweet chocolate chips with a rubber spatula or wooden spoon until evenly distributed.

Pour and spread the batter evenly into the prepared pan. Bake for 40–45 minutes, until a cake tester or toothpick inserted into the center comes out with a few moist crumbs but not wet batter. If the tester is coated with melted chocolate, test again in a different spot.

Transfer the pan to a wire rack and let the brownies cool in the pan for 20 minutes. Use the parchment overhang to lift the brownies out onto a cutting board. Slice into 16 squares or 12 rectangles. Dust with powdered sugar before serving, and serve slightly warm.