Prep and Preheat: Start by preheating your oven to 350°F (175°C). Grease and line a 9×13 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal. Combine Wet Ingredients: In a large microwave-safe bowl, melt the butter. Whisk in the granulated sugar and brown sugar until well combined. The mixture will be gritty. Add Eggs and Vanilla: Whisk in the eggs, one at a time, followed by the vanilla extract. Mix until the batter becomes smooth and glossy. Incorporate Dry Ingredients: Sift the cocoa powder, flour, and salt directly into the wet ingredients. Use a spatula to gently fold the dry ingredients into the wet until just combined. Be careful not to overmix; this is the secret to a fudgy, not cakey, texture. Bake to Perfection: Pour the batter into your prepared pan and spread it evenly. Bake for 28-32 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached. Do not overbake! Cool Completely: Let the brownies cool completely in the pan set on a wire rack. This is crucial before frosting. Make the Frosting: While the brownies cool, make the frosting. In a medium bowl, beat the softened butter with an electric mixer until smooth and creamy. Gradually add the powdered sugar, milk, vanilla, and salt. Beat on medium-high speed for 2-3 minutes until light and fluffy. Frost and Serve: Once the brownies are completely cool, spread the frosting evenly over the top. Lift the brownies out of the pan using the parchment paper and slice into squares.