Preheat oven to 350°F (175°C) and grease an 8×8-inch baking pan.

In a medium bowl, whisk melted butter with granulated and brown sugars until glossy.

Add eggs one at a time, mixing well after each addition. Stir in vanilla.

In another bowl, sift together cocoa powder, flour, and salt.

Gradually fold the dry ingredients into the wet mixture until just combined.

Pour batter into prepared pan and bake for 20 to 25 minutes, until a toothpick inserted comes out with moist crumbs (not wet batter).

Cool slightly before cutting into squares.