Preheat oven to 350°F. Butter and flour a 9x9-inch pan.

In a medium bowl, sift together cocoa and flour. Add salt and whisk to combine. Set aside.

Place half (12 tablespoons) of the butter in a medium bowl. In a small saucepan over medium heat, melt remaining half (12 tablespoons) of the butter. Pour melted butter over butter pieces and stir until melted, creamy, and room temperature.

In the bowl of a stand mixer fitted with the paddle attachment, beat eggs and sugar on medium until light and fluffy and pale, about 3 minutes.

With the mixer on low, add cocoa-flour mixture by thirds, alternating with thirds of the melted butter.

Add chocolate pieces and mix to combine.

Scrape batter into prepared pan and bake 40 to 45 minutes, or until a skewer inserted into the center comes out with a few crumbs attached.

Cool on a rack until slightly above room temperature. Run a knife around the edge, invert onto parchment or waxed paper, and slice into 12 pieces.

Serve with a dusting of powdered sugar.