Direction :

1. Preheat oven to 350°F and position a rack in the center of the oven. Line a 9-inch square baking pan, preferably aluminum or metal, with aluminum foil (see notes above).
2. In a large bowl, whisk together the cocoa, flour, baking powder, and salt. 
3. In another large bowl, beat the eggs very well until foamy and thick — about 1 minute of vigorous whisking. 
4. Melt the butter in a medium saucepan over medium heat — it should be hot and bubbling lightly. Add the sugar and whisk until well very combined — I like to use my flat-bottomed whisk here. The mixture should be nearly emulsified and should look like wet sand.
5. Pour the melted butter-sugar mix into the eggs and whisk constantly until very well combined. Be sure to scrape the bottom of the pot to extract any lingering sugar/butter. Add the vanilla to the egg-sugar mix, and whisk to incorporate.
6. Add the dry ingredients to the wet, and whisk until smooth. 
7. Spread the batter into the prepared pan and bake for approximately 37-40 minutes. Insert a toothpick straight into the center. If it comes out clean or with just a few moist crumbs, the brownies are done.
8. Let cool completely in pan on rack (at least 30 minutes) before cutting. Brownies can be made two to three days in advance.