Preheat Oven and Prepare Pan - Preheat your oven to 350 degrees Fahrenheit. Spray your 9-inch square baking pan generously with cooking spray.
Mix Together Butter, Sugar, and Vanilla - Using a microwave-safe bowl, melt your butter and set it aside for a couple of minutes so it can cool. In a large mixing bowl, whisk together the butter, sugar, and vanilla until smooth.
Add Eggs - Add the eggs into your butter mixture and beat well using a whisk or spoon.
Mix Flour, Cocoa Powder, Salt, and Baking Powder - In a medium bowl, mix together the flour, cocoa powder, salt, and baking powder.
Combine the Wet and Dry Ingredients of Your Brownie Batter - Slowly add the dry ingredients of the brownies into the wet ingredients to form a batter. Beat the mixture until well combined. If desired, add in chopped nuts of your choice.
Bake the Brownies - Spread the brownie batter evenly in your prepared pan. Bake the brownies for 20-25 minutes, until the brownies start to pull away from the pan. Allow the brownies to fully cool on a wire rack, which may take up to one hour.
Make the Frosting - In a medium bowl, beat the butter, cocoa, corn syrup, and vanilla until well combined. If desired you may use an electric mixer.
Add in the powdered sugar and milk, beating until the frosting is smooth and creamy. If needed, add more milk to reach a thicker consistency of your frosting.
Frist the Brownies - Using an offset spatula or rubber spatula, frost the brownies with the creamy frosting. Be sure the brownies are fully cool before frosting. If desired, top with chopped nuts or chocolate chips.