1 (level) cup unsweetened cocoa powder divided; 3/4 cup for brownies, 1/4 cup for frosting
1 cup unsalted butter, divided; 3/4 cup for brownies, 1/4 cup for frosting
2 cups granulated sugar
3 large eggs, lightly beaten
3 teaspoons vanilla extract, divided; 2 teaspoons for brownies, 1 teaspoon for frosting
1 cup all-purpose flour
3 tablespoons milk
1 teaspoon white vinegar
2-3 cups powdered sugar
1/2 teaspoon salt
Preheat oven to 350°F. Spray 9-by-11 inch baking dish with non-stick spray.
Microwave 3/4 cup butter in a large microwave-safe bowl in 10-second increments until melted. Add 3/4 cup cocoa powder and granulated sugar. Stir to combine.
Add eggs and 2 teaspoons vanilla. Stir to combine. Add flour and mix well.
Pour brownie batter into prepared pan and bake 25-30 minutes, or until toothpick inserted in center comes out clean.
Buttermilk frosting: combine milk and vinegar in a small saucepan. Allow to sit 5 minutes or until milk begins to curdle. Add reserved butter and cocoa powder. Boil mixture until butter is melted and cocoa is dissolved, about 10 minutes.
Add powdered sugar and reserved vanilla to frosting mixture. Whisk until thickened, about 5 minutes.
Allow brownies to cool completely. Spread frosting on brownies before serving.