Preheat your oven to 350°F. Prepare a 9x9-inch baking pan by lining it with parchment paper or aluminum foil, then lightly coat it with nonstick spray.

In a large bowl, blend the melted butter and sugar until the mixture becomes light and fluffy. Beat in the eggs and vanilla extract, mixing until fully incorporated.

Gradually fold in the flour and cocoa powder, stirring just enough to combine the ingredients. Pour the batter into the prepared baking pan, spreading it evenly.

Bake the brownies for 20-25 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs. Avoid overbaking.

While the brownies are baking, prepare the frosting. In a medium bowl, blend the softened butter, cocoa powder, milk, and powdered sugar until smooth. Adjust the consistency by adding extra milk if the frosting is too thick, or more powdered sugar if too thin.

Once the brownies are done baking, immediately spread the frosting on top while they are still warm. Allow the brownies to cool before cutting them into squares.