Preheat oven to 350°F (175°C) and grease an 8x8 inch baking pan with butter or parchment paper.

In a medium mixing bowl, combine melted butter and granulated sugar, whisking until well combined.

Add the eggs one at a time, beating well after each addition until the mixture is glossy.

Stir in the vanilla extract.

Sift the cocoa powder and salt into the wet ingredients and fold until just combined.

Transfer the batter to the prepared baking pan and smooth the top with a spatula.

Bake for 22 to 25 minutes, or until the edges are set but the center remains slightly soft.

Remove from oven and allow to cool completely in the pan to ensure the fudgy texture sets before slicing.