Preheat the Oven:
Preheat your oven to 350°F (175°C).

Make the Brownie Batter:
In a medium bowl, mix together the melted butter and cocoa powder until smooth. Add the flour, salt, and sugar, then stir until combined. The mixture will be dry and crumbly.

Add the Eggs and Vanilla:
Add in the eggs and vanilla extract and mix until smooth.

Prepare the Pan:
Line a 9×13-inch baking pan with parchment paper and pour the brownie batter into the pan, spreading it evenly.

Bake:
Bake for 20 to 25 minutes, or until a toothpick inserted in the middle comes out clean.

Make the Frosting:
While the brownies are baking, make the frosting by whisking together the melted butter, milk, powdered sugar, cocoa powder, and vanilla extract. The frosting should be thick and spreadable. If it’s too thick, add a little milk, one tablespoon at a time, until you achieve your desired consistency.

Cool the Brownies:
Once the brownies are baked, let them sit for 15 to 20 minutes before frosting.

Frost the Brownies:
Spread the frosting over the top of the brownies once they have cooled.

Let Cool and Serve:
Let the brownies cool completely before slicing into squares.