Unforgettable Chocolate Frosted Brownies

INGREDIENTS:

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

DIRECTIONS:

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 …