Batter Assembly: In a large mixing bowl, combine melted butter, granulated sugar, cocoa powder, and salt. Stir these ingredients until they are well blended and the mixture thickens. If the mixture is too warm, allow it to cool slightly. Incorporate the vanilla extract followed by the eggs, adding one egg at a time and mixing thoroughly after each addition. Once the mixture is smooth and well-blended, fold in the flour until just combined. Spread the batter evenly into the prepared pan.