8 (1 ounce) squares semisweet chocolate (such as Baker's®), chopped
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0.75 cup unsalted butter, cubed
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0.5 cup all-purpose flour
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0.5 cup unsweetened dark cocoa powder
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0.5 teaspoon salt
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6 large eggs, at room temperature
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1 cup white sugar
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1 cup packed brown sugar
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2 teaspoons vanilla extract
Instructions
Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan with shortening.
Combine chocolate and butter in a large, heavy saucepan over low heat, stirring constantly, until melted, about 5 minutes. Remove from heat.
Whisk flour, cocoa, and salt together in a bowl.
Beat eggs in a large bowl with an electric mixer on high speed until well blended. Gradually add white and brown sugars; beat until tripled in volume and very pale yellow, at least 10 minutes. The mixture should leave ribbons when you lift the beaters. Fold in the chocolate-butter mixture and vanilla extract, then flour mixture. Pour into the prepared pan.
Bake in the preheated oven until a toothpick inserted into the center comes out almost clean, 28 to 32 minutes. Cool before cutting into 16 brownies.