Ingredients 

340 grams butter
170 grams chocolate chips or dark chocolate (not unsweetened baking chocolate)
125 grams Soft White flour
115 grams Dutch-processed cocoa powder 
450 grams fine white sugar
55 grams brown sugar
½ tsp salt
6 cold eggs straight from the refrigerator 
1 tbsp vanilla

Makes one 9x13” pan

Adapted from Serious Eats blog by Stella Parks

Instructions

Melt the butter and chocolate  over low heat, then place the mixture into a measuring cup in the refrigerator to cool slightly.

Line a 9” x13” anondized aluminum pan with a parchment sling, cutting the parchment just above the sides of the pan. The type of pan is very important. Avoid using a glass or dark/non-stick pan as they will affect the texture of the brownies negatively.

Preheat the oven to 350 f.

Place the eggs, sugars, vanilla and salt in the bowl of a stand mixer fitted with the whisk attachment and whisk on medium high speed (#8 setting) for 8-10 minutes. Because timing can vary based on the power of the mixture and the ambient temperature, look for visual cues of doneness- the eggs should be pale, thick and fluffy. You want the maximum amount of air possible whipped into the eggs. 

Once the eggs are done, turn the speed down to low and add the warm melted butter/chocolate mixture. It doesn’t need to be well incorporated as the goal is to keep as much air in the eggs as possible.

Add the flour all at once and mix until just incorporated slightly. Finish mixing by hand; there can be large streaks of flour in the batter. Do not overmix. 

Pour the batter into the parchment-lined baking pan and smooth the top with a spatula. Place the pan in the center of the oven and bake for 35 minutes.

Place the pan on a cooling rack and let cool completely before using the parchment sling to remove the brownies from the pan, at least an hour. Slice and serve at room temperature.