“I’ve been doubling the recipe and baking in a 9×13 for years now.”

Preheat oven to 350˚F (175˚C).
Line a 9″x9″ baking pan with foil, ensuring the foil extends over the edges.
Make the Brownie Batter:

In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the eggs and vanilla extract, mixing until well combined.
Gradually add the cocoa powder and all-purpose flour, mixing until just combined and the batter is uniform in color.
Pour the batter into the prepared baking pan.
Bake the Brownies:

Bake for 20-25 minutes, or until a knife inserted into the center comes out with a few moist crumbs.
Avoid overbaking for the best texture.
Allow brownies to begin to cool.
Prepare the Frosting:

While the brownies are cooling, in a medium bowl, combine the softened butter, milk, cocoa powder, and powdered sugar.
Mix until smooth and creamy.
If the frosting is too dry, add milk one tablespoon at a time. If it is too thin, add powdered sugar one tablespoon at a time.
Frost and Serve:

Spread the frosting over the warm brownies, allowing some of it to melt into the surface.
Cut the brownies into squares.
Use the foil to lift the brownies out of the pan for easier serving.