Step 1

For the brownies, preheat the oven to 400 degrees F. Butter a 9 by 13-inch baking pan with softened butter; set the pan aside.

Step 3

Heat the butter, cocoa powder, vegetable oil, and water in a medium saucepan over medium-high heat. Bring to a boil. Remove from the heat and allow to cool just until warm; set aside.

Step 5

Sift the flour, baking soda, and salt into a medium bowl; set aside.

Step 7

Combine the granulated sugar, eggs, buttermilk, and vanilla extract in a large bowl. Whisk to combine. Add the cooled cocoa mixture.

Step 9

Add the dry ingredients to the wet ingredients and stir until just combined. Pour the batter into the prepared baking pan. Bake for about 20 minutes or until a cake tester inserted into the center comes out clean.

Step 11

For the frosting, combine the butter, buttermilk, and cocoa powder in a saucepan. Bring to a boil over medium-high heat. Remove from the heat.

Step 13

Combine the powdered sugar, vanilla extract, and salt in a large mixing bowl. Slowly add the buttermilk mixture to the sugar mixture, beating with an electric mixer on medium speed. Continue beating until smooth and creamy. Allow to cool completely.

Step 15

After the brownies have cooled, spread the frosting evenly over the top.