Preheat and prep the pan. Heat the oven to 350°F (175°C). Grease a 9x13-inch baking pan and line with parchment, leaving a slight overhang for easy lifting.
Melt the butter. Melt the butter on the stove or in the microwave until fully liquid but not sizzling. Let it cool for 2–3 minutes so it doesn’t scramble the eggs.
Mix the wet ingredients. In a large bowl, whisk together melted butter, granulated sugar, eggs, and vanilla until smooth and glossy, about 1–2 minutes.
Add the dry ingredients. Sift in cocoa powder, flour, and salt.
Stir gently with a spatula until just combined. Do not overmix.
Fold in extras. If using nuts or chocolate chips, fold them in now. The batter will be thick—that’s perfect.
Bake. Spread the batter evenly in the pan.
Bake for 20–25 minutes, until the center is set and a toothpick comes out with moist crumbs (not wet batter). Do not overbake.
Make the icing. While the brownies bake, whisk together powdered sugar, cocoa powder, and milk until smooth and pourable. Add a splash more milk if needed.
The icing should be thick but spreadable.
Ice while warm. As soon as the brownies come out, pour the icing over the warm surface. Spread gently to the edges. The warmth helps the icing settle into that classic shiny layer.
Cool and slice. Let the brownies cool completely in the pan for clean slices.
Use the parchment to lift out and cut into squares.